Developmental plasticity in the respiratory pathway of the adult rat

Effects of C2 hemisection on respiratory and cardiovascular functions in rats

High cervical spinal cord injuries induce permanent neuromotor and autonomic deficits. These injuries impact both central respiratory and cardiovascular functions through modulation of the sympathetic nervous system. So far, cardiovascular studies have focused on models of complete contusion or transection at the lower cervical and thoracic levels and diaphragm activity evaluations using invasive methods. The present study aimed to evaluate the impact of C2 hemisection on different parameters representing vital functions (i.e., respiratory function, cardiovascular, and renal filtration parameters) at the moment of injury and 7 days post-injury in rats. No ventilatory parameters evaluated by plethysmography were impacted during quiet breathing after 7 days post-injury, whereas permanent diaphragm hemiplegia was observed by ultrasound and confirmed by diaphragmatic electromyography in anesthetized rats. Interestingly, the mean arterial pressure was reduced immediately after C2 hemisection, with complete compensation at 7 days post-injury. Renal filtration was unaffected at 7 days post-injury; however, remnant systolic dysfunction characterized by a reduced left ventricular ejection fraction persisted at 7 days post-injury. Taken together, these results demonstrated that following C2 hemisection, diaphragm activity and systolic function are impacted up to 7 days post-injury, whereas the respiratory and cardiovascular systems display vast adaptation to maintain ventilatory parameters and blood pressure homeostasis, with the latter likely sustained by the remaining descending sympathetic inputs spared by the initial injury. A better broad characterization of the physiopathology of high cervical spinal cord injuries covering a longer time period post-injury could be beneficial for understanding evaluations of putative therapeutics to further increase cardiorespiratory recovery.

Plasticity in Cervical Motor Circuits following Spinal Cord Injury and Rehabilitation

Simple Summary

Spinal cord injury results in a decreased quality of life and impacts hundreds of thousands of people in the US alone. This review discusses the underlying cellular mechanisms of injury and the concurrent therapeutic hurdles that impede recovery. It then describes the phenomena of neural plasticity—the nervous system’s ability to change. The primary focus of the review is on the impact of cervical spinal cord injury on control of the upper limbs. The neural plasticity that occurs without intervention is discussed, which shows new connections growing around the injury site and the involvement of compensatory movements. Rehabilitation-driven neural plasticity is shown to have the ability to guide connections to create more normal functions. Various novel stimulation and recording technologies are outlined for their role in further improving rehabilitative outcomes and gains in independence. Finally, the importance of sensory input, an often-overlooked aspect of motor control, is shown in driving neural plasticity. Overall, this review seeks to delineate the historical and contemporary research into neural plasticity following injury and rehabilitation to guide future studies.

Abstract

Neuroplasticity is a robust mechanism by which the central nervous system attempts to adapt to a structural or chemical disruption of functional connections between neurons. Mechanical damage from spinal cord injury potentiates via neuroinflammation and can cause aberrant changes in neural circuitry known as maladaptive plasticity. Together, these alterations greatly diminish function and quality of life. This review discusses contemporary efforts to harness neuroplasticity through rehabilitation and neuromodulation to restore function with a focus on motor recovery following cervical spinal cord injury. Background information on the general mechanisms of plasticity and long-term potentiation of the nervous system, most well studied in the learning and memory fields, will be reviewed. Spontaneous plasticity of the nervous system, both maladaptive and during natural recovery following spinal cord injury is outlined to provide a baseline from which rehabilitation builds. Previous research has focused on the impact of descending motor commands in driving spinal plasticity. However, this review focuses on the influence of physical therapy and primary afferent input and interneuron modulation in driving plasticity within the spinal cord. Finally, future directions into previously untargeted primary afferent populations are presented.

Spontaneous respiratory plasticity following unilateral high cervical spinal cord injury in behaving rats

Unilateral cervical C2 hemisection (C2Hx) is a classic model of spinal cord injury (SCI) for studying respiratory dysfunction and plasticity. However, most previous studies were performed under anesthesia, which significantly alters respiratory network. Therefore, the goal of this work was to assess spontaneous diaphragm recovery post-C2Hx in awake, freely behaving animals. Adult rats were chronically implanted with diaphragm EMG electrodes and recorded during 8 weeks post-C2Hx. Our results reveal that ipsilateral diaphragm activity partially recovers within days post-injury and reaches pre-injury amplitude in a few weeks. However, the full extent of spontaneous ipsilateral recovery is significantly attenuated by anesthesia (ketamine/xylazine, isoflurane, and urethane). This suggests that the observed recovery may be attributed in part to activation of NMDA receptors which are suppressed by anesthesia. Despite spontaneous recovery in awake animals, ipsilateral hemidiaphragm dysfunction still persists: i) Inspiratory bursts during basal (slow) breathing exhibit an altered pattern, ii) the amplitude of sighs - or augmented breaths - is significantly decreased, and iii) the injured hemidiaphragm exhibits spontaneous events of hyperexcitation. The results from this study offer an under-appreciated insight into spontaneous diaphragm activity and recovery following high cervical spinal cord injury in awake animals.

Phrenic motoneurons: output elements of a highly organized intraspinal network

pontomedullary respiratory network generates the respiratory pattern and relays it to bulbar and spinal respiratory motor outputs. The phrenic motor system controlling diaphragm contraction receives and processes descending commands to produce orderly, synchronous, and cycle-to-cycle-reproducible spatiotemporal firing. Multiple investigators have studied phrenic motoneurons (PhMNs) in an attempt to shed light on local mechanisms underlying phrenic pattern formation. I and colleagues (Marchenko V, Ghali MG, Rogers RF. Am J Physiol Regul Integr Comp Physiol 308: R916–R926, 2015.) recorded PhMNs in unanesthetized, decerebrate rats and related their activity to simultaneous phrenic nerve (PhN) activity by creating a time-frequency representation of PhMN-PhN power and coherence. On the basis of their temporal firing patterns and relationship to PhN activity, we categorized PhMNs into three classes, each of which emerges as a result of intrinsic biophysical and network properties and organizes the orderly contraction of diaphragm motor fibers. For example, early inspiratory diaphragmatic activation by the early coherent burst generated by high-frequency PhMNs may be necessary to prime it to overcome its initial inertia. We have also demonstrated the existence of a prominent role for local intraspinal inhibitory mechanisms in shaping phrenic pattern formation. The objective of this review is to relate and synthesize recent findings with those of previous studies with the aim of demonstrating that the phrenic nucleus is a region of active local processing, rather than a passive relay of descending inputs.

The bulbospinal network controlling the phrenic motor system: Laterality and course of descending projections

The respiratory rhythm is generated by the parafacial respiratory group, Bötzinger complex, and pre-Bötzinger complex and relayed to pre-motor neurons, which in turn project to and control respiratory motor outputs in the brainstem and spinal cord. The phrenic nucleus is one such target, containing phrenic motoneurons (PhMNs), which supply the diaphragm, the primary inspiratory muscle in mammals. While some investigators have demonstrated both ipsi- and contralateral bulbophrenic projections, there exists controversy regarding the relative physiological contribution of each to phasic and tonic drive to PhMNs and at which levels decussations occur. Following C1- or C2 spinal cord hemisection-induced silencing of the ipsilateral phrenic/diaphragm activity, respiratory stressor-induced, as well as spontaneous, recovery of crossed phrenic activity is observed, suggesting an important contribution of pathways crossing below the level of injury in driving phrenic motor output. The precise mechanisms underlying this recovery are debated. In this review, we seek to present a comprehensive discussion of the organization of the bulbospinal network controlling PhMNs, a thorough appreciation of which is necessary for understanding neural respiratory control, accurate interpretation of studies investigating respiratory recovery following spinal cord injury, and targeted development of therapies for respiratory neurorehabilitation in patients sustaining high cervical cord injury.

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.

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).

Clinical Effects of Theophylline on Inspiratory Muscle Drive in Tetraplegia

Theophylline has been shown to restore diaphragmatic function in animals following cervical spinal cord hemisection, which induces hemidiaphragm paralysis. Although theophylline had been used clinically in the treatment of various pulmonary diseases, its effects on respiratory muscle function in cervical spinal cord injured tetraplegics has not been studied. In the present case study, we evaluated a patient injured in 1979 with a chronic asymmetric C5-7 tetraplegia (left C5-6, right C6-7) before and after receiving theophylline chronically by mouth for three weeks and again before and after receiving acute intravenous (IV) aminophylline after the effects of the chronic drug administration wore off. Neural activation to inspiratory muscles was assessed by right and left parasternal intercostal and diaphragm EMGs during quiet breathing and max imal inspiratory efforts. Global respiratory drive was assessed by P100, and inspiratory muscle force was assessed by maximal inspiratory pressures and vital capacity. Both long-term orally administered and acute IV theophylline increased neural activation to the diaphragm, especially on the more affected left side. Theophylline treatment was also associated with an increase in global central respiratory drive and inspiratory muscle force, without changing expiratory airflows. Left diaphragm EMG activity was markedly increased following the administration of theophylline. Of interest, upper parasternal intercostal EMG activity was also recruited on the left in spite of being below the level of cervical injury. We speculate that the administration of theophylline in selected patients with an asymmetric cervical spinal cord injury may activate la tent bulbospinal respiratory pathways and improve inspiratory muscle function, re ducing the likelihood of associated respiratory failure.

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.

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.

Identification of the neural pathway underlying spontaneous crossed phrenic activity in neonatal rats

Cervical spinal cord hemisection at C2 leads to paralysis of the ipsilateral hemidiaphragm in rats. Respiratory function of the paralyzed hemidiaphragm can be restored by activating a latent respiratory motor pathway in adult rats. This pathway is called the crossed phrenic pathway and the restored activity in the paralyzed hemidiaphragm is referred to as crossed phrenic activity. The latent neural pathway is not latent in neonatal rats as shown by the spontaneous expression of crossed phrenic activity. However, the anatomy of the pathway in neonatal rats is still unknown. In the present study, we hypothesized that the crossed phrenic pathway may be different anatomically in neonatal and adult rats. To delineate this neural pathway in neonates, we injected wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), a retrograde transsynaptic tracer, into the phrenic nerve ipsilateral to hemisection. We also injected cholera toxin subunit B-horseradish peroxidase (BHRP) into the ipsilateral hemidiaphragm following hemisection in other animals to determine if there are midline-crossing phrenic dendrites involved in the crossed phrenic pathway in neonatal rats. The WGA-HRP labeling was observed only in the ipsilateral phrenic nucleus and ipsilateral rostral ventral respiratory group (rVRG) in the postnatal day (P) 2, P7, and P28 hemisected rats. Bilateral labeling of rVRG neurons was shown in P35 rats. The BHRP study showed that many phrenic dendrites cross the midline in P2 neonatal rats at both rostral and caudal parts of the phrenic nucleus. There was a marked reduction of crossing dendrites observed in P7 and P28 animals and no crossing dendrites observed in P35 rats. The present results suggest that the crossed phrenic pathway in neonatal rats involves the parent axons from ipsilateral rVRG premotor neurons that cross at the level of obex as well as decussating axon collaterals that cross over the spinal cord midline to innervate ipsilateral phrenic motoneurons following C2 hemisection. In addition, midline-crossing dendrites of the ipsilateral phrenic motoneurons may also contribute to the crossed phrenic pathway in neonates.

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.

The potential role of phrenic nucleus glutamate receptor subunits in mediating spontaneous crossed phrenic activity in neonatal rat

Cervical spinal cord hemisection rostral to the phrenic nucleus leads to paralysis of the ipsilateral hemidiaphragm in adult rats. Respiratory function can be restored to the paralyzed hemidiaphragm by activating a latent respiratory motor pathway. The latent pathway is called the crossed phrenic pathway. In adult rats, the pathway can be activated by drug-induced upregulation of NMDA receptor NR2A subunit and AMPA receptor GluR1 subunit in the phrenic nucleus following hemisection. In neonatal rats, this pathway is not latent as shown by the spontaneous expression of activity in the ipsilateral hemidiaphragm following hemisection. We hypothesized that the NR2A and GluR1 subunits may be highly expressed naturally on phrenic motoneurons of neonatal rats and may play a potential role in mediating the spontaneous expression of activity in the ipsilateral hemidiaphragm after hemisection. To test this hypothesis, the protein levels of NR2A and GluR1 in different age rats were assessed via Western blot analysis immediately following C2 hemisection and EMG recording of crossed phrenic activity. The protein levels of NR2A and GluR1 were transiently high in postnatal day 2 (P2) rats and then was significantly reduced in P7 and P35 animals. An immunofluorescence study qualitatively supported these findings. The present results indicate that the developmental downregulation of the phrenic nucleus glutamate receptor subunits correlates with the conversion of the crossed phrenic pathway in older postnatal animals from an active state to a latent state.

Postnatal conversion of cross phrenic activity from an active to latent state

Spinal cord hemisection rostral to the phrenic nucleus leads to paralysis of the ipsilateral hemidiaphragm and respiratory insufficiency. Recovery of the paralyzed hemidiaphragm may be induced by activating a latent respiratory motor pathway in adult rats. Although the pathway is latent in adults, it may not be latent in neonatal rats as shown by the spontaneous expression of activity over this pathway in an earlier in vitro study. Activity mediated over the latent pathway is known as "crossed phrenic activity". Whether crossed phrenic activity following C2 spinal cord hemisection occurs spontaneously in the neonatal rat in vivo is still unknown. We hypothesized that crossed phrenic activity may be spontaneously expressed in neonates in vivo and may be converted from a spontaneously active state to a latent and nonfunctional state during postnatal development. Thus, a time course study was designed to analyze this activity in rat pups at different ages. The functional status of the ipsilateral and contralateral hemidiaphragms was tested by EMG analysis following hemisection. Crossed phrenic activity was expressed in ventral, lateral, and dorsal parts of the ipsilateral hemidiaphragm in P2 and some P3 and P4 neonatal rats. During postnatal development, the activity was observed only in the ventral area of the ipsilateral hemidiaphragm in P7, P14, P21 and P28 animals. Significant decreases in the extent of ventral crossed phrenic activity were observed from P2 to P28. The pathway generating this activity becomes latent by postnatal day 35. The present results suggest that spontaneous crossed phrenic activity occurs in vivo following C2 hemisection and the activity gradually decreases during the first four postnatal weeks.

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.

Plasminogen activator induction facilitates recovery of respiratory function following spinal cord injury

The possibility that plasminogen activator (PA) plays a role in synaptic plasticity was explored in the spinal cord during the crossed phrenic phenomenon (CPP), where respiratory functional plasticity develops following spinal cord injury. Synaptic remodeling on phrenic motorneurons occurs during the characteristic delay period following spinal cord injury before CPP recovery of respiratory function. The molecular mechanisms underlying this plasticity are not well-defined. During the critical 1-2 h delay period required for this synaptic plasticity following a C2 hemisection in mice, uPA and tPA mRNAs are rapidly induced in C4-5 ventral spinal cord neurons in the ipsilateral phrenic motor nucleus (PMN), as are uPA and tPA protein levels. A role for uPA in CPP spinal cord plasticity is confirmed by the impaired ability of uPA knockout mice to acquire a good CPP response by 6 h post-hemisection and their lack of structural remodeling of PMN synapses that underlies development of the CPP response.

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.

Differential expression of adenosine A1 and A2A receptors after upper cervical (C2) spinal cord hemisection in adult rats

BACKGROUND

In an animal model of spinal cord injury, a latent respiratory motor pathway can be pharmacologically activated via adenosine receptors to restore respiratory function after cervical (C2) spinal cord hemisection that paralyzes the hemidiaphragm ipsilateral to injury. Although spinal phrenic motoneurons immunopositive for adenosine receptors have been demonstrated (C3-C5), it is unclear if adenosine receptor protein levels are altered after C2 hemisection and theophylline administration.

OBJECTIVE

To assess the effects of C2 spinal cord hemisection and theophylline administration on the expression of adenosine receptor proteins.

METHODS

Adenosine A1 and A2A receptor protein levels were assessed in adult rats classified as (a) noninjured and theophylline treated, (b) C2 hemisected, (c) C2 hemisected and administered theophylline orally (3x daily) for 3 days only, and (d) C2 hemisected and administered theophylline (3x daily for 3 days) and assessed 12 days after drug administration. Assessment of A1 protein levels was carried out via immunohistochemistry and A2A protein levels by densitometry.

RESULTS

Adenosine A1 protein levels decreased significantly (both ipsilateral and contralateral to injury) after C2 hemisection; however, the decrease was attenuated in hemisected and theophylline-treated animals. Attenuation in adenosine A1 receptor protein levels persisted when theophylline administration was stopped for 12 days prior to assessment. Adenosine A2A protein levels were unchanged by C2 hemisection; however, theophylline reduced the levels within the phrenic motoneurons. Furthermore, the decrease in A2A levels persisted 12 days after theophylline was withdrawn.

CONCLUSION

Our findings suggest that theophylline mitigates the effects of C2 hemisection by attenuating the C2 hemisection-induced decrease in A1 protein levels. Furthermore, A2A protein levels are unaltered by C2 hemisection but decrease after continuous or interrupted theophylline administration. The effects on protein levels may underlie the stimulant actions of theophylline.

Spinal cord injury-induced plasticity in the mouse--the crossed phrenic phenomenon

The crossed phrenic phenomenon (CPP) describes respiratory functional plasticity that arises following spinal cord injury. Cervical spinal cord hemisection rostral to the phrenic nucleus paralyzes the ipsilateral hemidiaphragm by interrupting the descending flow of respiratory impulses from the medulla to phrenic motoneurons in the spinal cord. This loss of activity converts some synapses on phrenic motoneurons from a "functionally ineffective" state pre-hemisection to a "functionally latent" state post-hemisection. If the animal is subjected to respiratory stress by transecting the contralateral phrenic nerve, this latent respiratory pathway is activated and function is restored to the paralyzed hemidiaphragm. The mechanisms underlying this plasticity are not well-defined, particularly at the molecular level. Therefore, we explored whether it was possible to demonstrate the CPP in mice, a species amenable to a molecular genetic approach. We show the CPP qualitatively in mice using electromyographic (EMG) recordings from the diaphragm. Interestingly, our data also suggest that in the mouse latent fibers in the ventral funiculus ipsilateral to an anatomically incomplete hemisection may also play a role in the CPP. In particular, we examined the inter-operative delay time between the spinal cord injury and contralateral phrenicotomy required for a response. As the inter-operative delay was reduced, the proportion of mice displaying the CPP decreased from 95% for overnight animals, 86% in 4-8 h, to 77% for 1-2 h mice, and less than 28% for animals receiving a phrenicotomy under 0.5 h post-spinal cord lesion. This is the first study to demonstrate the CPP in mice.

Spinal synaptic enhancement with acute intermittent hypoxia improves respiratory function after chronic cervical spinal cord injury

Respiratory insufficiency is the leading cause of death after high-cervical spinal cord injuries (SCIs). Although respiratory motor recovery can occur with time after injury, the magnitude of spontaneous recovery is limited. We hypothesized that partial respiratory motor recovery after chronic cervical SCI could be strengthened using a known stimulus for spinal synaptic enhancement, intermittent hypoxia. Phrenic motor output was recorded before and after intermittent hypoxia from anesthetized, vagotomized, and pump-ventilated control and C2 spinally hemisected rats at 2, 4, and 8 weeks after injury. Weak spontaneous phrenic motor recovery was present in all C2-injured rats via crossed spinal synaptic pathways that convey bulbospinal inspiratory premotor drive to phrenic motoneurons on the side of injury. Intermittent hypoxia augmented crossed spinal synaptic pathways [phrenic long-term facilitation; pLTF] for up to 60 min after hypoxia at 8 weeks, but not 2 weeks, after injury. Ketanserin, a serotonin 2A receptor antagonist, administered before intermittent hypoxia at 8 weeks after injury prevented pLTF. Serotonergic innervation near phrenic motoneurons was assessed after injury. The limited magnitude of pLTF at 2 weeks was associated with an injury-induced reduction in serotonin-containing nerve terminals in the vicinity of phrenic motoneurons ipsilateral to C2 hemisection. Thereafter, pLTF magnitude progressively increased with the recovery of serotonergic innervation in the phrenic motor nucleus. Intermittent hypoxia (or pLTF) has intriguing possibilities as a therapeutic tool, because its greatest efficacy may be in patients with chronic SCI, a time when most patients have already achieved maximal spontaneous functional recovery.

Involvement of peripheral adenosine A2 receptors in adenosine A1 receptor-mediated recovery of respiratory motor function after upper cervical spinal cord hemisection

BACKGROUND/OBJECTIVE

In an animal model of spinal cord injury, a latent respiratory motor pathway can be pharmacologically activated through central adenosine A1 receptor antagonism to restore respiratory function after cervical (C2) spinal cord hemisection that paralyzes the hemidiaphragm ipsilateral to injury. Although respiration is modulated by central and peripheral mechanisms, putative involvement of peripheral adenosine A2 receptors in functional recovery in our model is untested. The objective of this study was to assess the effects of peripherally located adenosine A2 receptors on recovery of respiratory function after cervical (C2) spinal cord hemisection.

METHODS

Respiratory activity was electrophysiologically assessed (under standardized recording conditions) in C2-hemisected adult rats with the carotid bodies intact (H-CBI; n=12) or excised (H-CBE; n=12). Animals were administered the adenosine A2 receptor agonist, CGS-21680, followed by the A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), or administered DPCPX alone. Recovered respiratory activity, characterized as drug-induced activity in the previously quiescent left phrenic nerve of C2-hemisected animals in H-CBI and H-CBE rats, was compared. Recovered respiratory activity was calculated by dividing drug-induced activity in the left phrenic nerve by activity in the right phrenic nerve.

RESULTS

Administration of CGS-21680 before DPCPX (n=6) in H-CBI rats induced a significantly greater recovery (58.5 +/- 3.6%) than when DPCPX (42.6 +/- 4.6%) was administered (n=6) alone. In H-CBE rats, prior administration of CGS-21680 (n=6) did not enhance recovery over that induced by DPCPX (n=6) alone. Recovery in H-CBE rats amounted to 39.7 +/- 3.7% and 38.4 + 4.2%, respectively.

CONCLUSIONS

Our results suggest that adenosine A2 receptors located in the carotid bodies can enhance the magnitude of adenosine A1 receptor-mediated recovery of respiratory function after C2 hemisection. We conclude that a novel approach of targeting peripheral and central adenosine receptors can be therapeutically beneficial in alleviating compromised respiratory function after cervical spinal cord injury.

Effects of carotid body excision on recovery of respiratory function in C2 hemisected adult rats

In a previous study, we described the spontaneous recovery of respiratory motor function in adult rats subjected to a left C2 hemisection 6-16 weeks post-injury without any therapeutic intervention. We extend the previous findings by demonstrating in the present study that rats subjected to a left C2 hemisection with bilateral carotid body excision will also recover respiratory-related activity in the paralyzed ipsilateral hemidiaphragm. However, in this instance, recovery is significantly accelerated; i.e., it is evident as early as 2 weeks after spinal cord injury. Two experimental groups (and noninjured and sham-operated controls) of rats were employed in the study. H-CBE animals were subjected to a left C2 hemisection plus bilateral carotid body excision while H-CBI animals were subjected to a left C2 hemisection only. Carotid body excision was confirmed by the sodium cyanide test. The animals were allowed to survive for 2 weeks after hemisection. Thereafter, electrophysiologic assessment of respiratory activity was conducted in all animals. Spontaneous recovery of respiratory-related activity in the paralyzed hemidiaphragm (indicated by left phrenic nerve activity) was detected in all H-CBE animals while H-CBI animals did not express spontaneous recovery of diaphragmatic activity. The magnitude of recovered activity when expressed as a function of contralateral phrenic nerve activity was 48.8 +/- 3.8%. When expressed as a function of the homolateral phrenic nerve in noninjured animals, the magnitude amounted to 25.6 +/- 2.8%. Although the mechanisms responsible for the apparent early onset of spontaneous recovery are unknown, it is likely that a reorganization of the respiratory circuitry in the CNS may be involved. The significance of the findings is that it may be feasible to modulate the onset of functional recovery following cervical spinal cord injury by specifically targeting peripheral chemoreceptors.

Adenosinergic mechanisms underlying recovery of diaphragm motor function following upper cervical spinal cord injury: potential therapeutic implications

OBJECTIVES

In adult rats, a latent respiratory motor pathway can be pharmacologically activated with 1,3-dimethylxanthine (theophylline) to restore respiratory-related activity to a hemidiaphragm paralysed by an ipsilateral upper cervical (C2) spinal cord hemisection. The purpose of this review is to describe mechanisms that underlie theophylline-induced recovery of respiratory-related function following C2 hemisection and to underscore the therapeutic potential of theophylline therapy in spinal cord injured patients with respiratory deficits.

METHODS

Theophylline mediates recovery of respiratory-related activity via antagonism of central adenosine A(1) receptors. When administered chronically, the drug restores and maintains recovered function. Since theophylline is an adenosine receptor antagonist with affinity for both the adenosine A(1) and A(2) receptors, we assessed the relative contributions of each receptor to functional recovery. While A(1) receptor antagonism plays a predominant role, activation of the A(2) receptors by specific agonists subserves the A(1) receptor-mediated actions. That is, when an adenosine A(2) receptor agonist is administered first, it primes the system such that subsequent administration of the A(1) antagonist induces a greater degree of recovered respiratory activity than when the antagonist alone is administered.

RESULTS

Chronic oral administration of theophylline in C2 hemisected animals demonstrates that even when animals have been weaned from the drug, theophylline-induced recovered respiratory actions persist. This suggests that in clinical application, it may not be necessary to maintain patients on long-term theophylline. We have shown that recovery of respiratory-related activity in the ipsilateral phrenic nerve can occur spontaneously 3-4 months after C2 hemisection. Theophylline administration after this post-injury period obliterates/negates the recovery function. This indicates strongly that there is therapeutic window (more acutely after injury) for the initiation of theophylline therapy. We have also demonstrated that peripheral (carotid bodies) adenosine A(1) receptors can be selectively activated to modulate theophylline-induced CNS actions. Blocking central adenosine receptors while simultaneously activating peripheral adenosine receptors minimizes the potential of respiratory muscle fatigue with theophylline.

DISCUSSION

The significance of the current findings lies in the potential clinical application of theophylline therapy in spinal cord injured patients with respiratory deficits. The ultimate goal of theophylline therapy is to wean ventilator-dependent patients off ventilatory support. Thus far, our animal studies suggest that the onset of theophylline therapy must be soon after injury.

Recovery of respiratory function following C2 hemi and carotid body denervation in adult rats: influence of peripheral adenosine receptors

The efficacy of the methylxanthine, theophylline, as a respiratory stimulant has been demonstrated previously in an animal model of spinal cord injury. In this model, an upper cervical (C2) spinal cord hemi paralyzes the ipsilateral hemidiaphragm. Theophylline restores respiratory-related activity in the paralyzed hemidiaphragm via activation of a latent respiratory motor pathway. Antagonism of central adenosine A1 receptors mediates this action. Theophylline also enhances respiratory frequency, f, defined as breaths per minute. Thus, long-term use may result in respiratory muscle or motoneuron fatigue particularly after spinal cord injury. We assessed the effects of an adenosine A1 receptor agonist, N6-p-sulfophenyladenosine (p-SPA) on theophylline's action in our model under standardized recording conditions. Four groups of rats, classified as hemisected/nonhemisected with the carotid bodies denervated (H-CBD or NH-CBD), and hemisected/nonhemisected with the carotid bodies intact (H-CBI or NH-CBI ) were used in the study. Eight days after recovery from carotid denervation, a left C2 hemi was performed in H-CBD rats. C2 hemi was also performed in H-CBI animals, and 24 h later, electrophysiologic experiments on respiratory activity were conducted in both groups of animals. Two groups using nonhemisected controls were also employed as described above. In H-CBD rats, theophylline significantly (P < 0.05) enhanced f and induced respiratory-related activity in the previously quiescent left phrenic nerve. In NH-CBD rats, theophylline significantly enhanced f. In both H-CBD and NH-CBD rats, p-SPA (0.25 mg/kg) did not significantly change theophylline-induced effects. In H-CBI rats, theophylline significantly (P < 0.05) enhanced f and induced activity in the previously quiescent left phrenic nerve. In H-CBI rats, p-SPA reduced the values to pre-theophylline discharge levels. Recovered activity was not obliterated with the agonist. In NH-CBI rats, p-SPA reduced theophylline-induced effects to pre-drug discharge levels. Adenosine A1 and A2A receptor immunoreactivity was detected in the carotid bodies. The significance of our findings is that theophylline-induced effects can be normalized to pre-drug levels by the selective activation of peripheral adenosine A1 receptors. The therapeutic benefits of theophylline, i.e., recovered respiratory function after paralysis, however, persists. The potential therapeutic impact is that respiratory muscle fatigue associated with long-term theophylline use may be minimized by a novel therapeutic approach.

Respiratory motor recovery after unilateral spinal cord injury: eliminating crossed phrenic activity decreases tidal volume and increases contralateral respiratory motor output

By 2 months after unilateral cervical spinal cord injury (SCI), respiratory motor output resumes in the previously quiescent phrenic nerve. This activity is derived from bulbospinal pathways that cross the spinal midline caudal to the lesion (crossed phrenic pathways). To determine whether crossed phrenic pathways contribute to tidal volume in spinally injured rats, spontaneous breathing was measured in anesthetized C2 hemisected rats at 2 months after injury with an intact ipsilateral phrenic nerve, or with ipsilateral phrenicotomy performed at the time of the SCI (i.e., crossed phrenic pathways rendered ineffective) (dual injury). Ipsilateral phrenicotomy did not alter the rapid shallow eupneic breathing pattern in C2 injured rats. However, the ability to generate large inspiratory volumes after either vagotomy or during augmented breaths was impaired if crossed phrenic activity was abolished. We also investigated whether compensatory plasticity in contralateral motoneurons would be affected by eliminating crossed phrenic activity. Thus, contralateral phrenic motor output was recorded in anesthetized, vagotomized, and mechanically ventilated rats with dual injury during chemoreceptor stimulation. Hypercapnia, hypoxia, and asphyxia increased contralateral phrenic burst amplitude in the dual injury group more than in rats with SCI alone. Dual injury rats also had elevated baseline burst frequency. Together, these results demonstrate a functional role of crossed phrenic activity after SCI. Moreover, by preventing ipsilateral phrenic motor recovery in rats with unilateral SCI, segmental and supraspinal changes could be induced in contralateral respiratory motor output beyond that seen with SCI alone.

The crossed phrenic phenomenon: a model for plasticity in the respiratory pathways following spinal cord injury

Hemisection of the cervical spinal cord rostral to the level of the phrenic nucleus interrupts descending bulbospinal respiratory pathways, which results in a paralysis of the ipsilateral hemidiaphragm. In several mammalian species, functional recovery of the paretic hemidiaphragm can be achieved by transecting the contralateral phrenic nerve. The recovery of the paralyzed hemidiaphragm has been termed the "crossed phrenic phenomenon." The physiological basis for the crossed phrenic phenomenon is as follows: asphyxia induced by spinal hemisection and contralateral phrenicotomy increases central respiratory drive, which activates a latent crossed respiratory pathway. The uninjured, initially latent pathway mediates the hemidiaphragm recovery by descending into the spinal cord contralateral to the hemisection and then crossing the midline of the spinal cord before terminating on phrenic motoneurons ipsilateral and caudal to the hemisection. The purpose of this study is to review work conducted on the crossed phrenic phenomenon and to review closely related studies focusing particularly on the plasticity associated with the response. Because the review deals with recovery of respiratory muscles paralyzed by spinal cord injury, the clinical relevance of the reviewed studies is highlighted.

Plasticity in the control of breathing following sensory denervation

The purpose of this manuscript is to review the results of studies on the recovery or plasticity following a denervation- or lesion-induced change in breathing. Carotid body denervation (CBD), lung denervation (LD), cervical (CDR) and thoracic (TDR) dorsal rhizotomy, dorsal spinal column lesions, and lesions at pontine, medullary, and spinal sites all chronically alter breathing. The plasticity after these is highly variable, ranging from near complete recovery of the peripheral chemoreflex in rats after CBD to minimal recovery of the Hering-Breuer inflation reflex in ponies after LD. The degree of plasticity varies among the different functions of each pathway, and plasticity varies with the age of the animal when the lesion was made. In addition, plasticity after some lesions varies between species, and plasticity is greater in the awake than in the anesthetized state. Reinnervation is not a common mechanism of plasticity. There is evidence supporting two mechanisms of plasticity. One is through upregulation of an alternate sensory pathway, such as serotonin-mediated aortic chemoreception after CBD. The second is through upregulation on the efferent limb of a reflex, such as serotonin-mediated increased responsiveness of phrenic motoneurons after CDR, TDR, and spinal cord injury. Accordingly, numerous components of the ventilatory control system exhibit plasticity after denervation or lesion-induced changes in breathing; this plasticity is uniform neither in magnitude nor in underlying mechanisms. A major need in future research is to determine whether "reorganization" within the central nervous system contributes to plasticity following lesion-induced changes in breathing.

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.

Actions of specific adenosine receptor A1 and A2 agonists and antagonists in recovery of phrenic motor output following upper cervical spinal cord injury in adult rats

1. Previous studies from our laboratory have established that a latent respiratory motor pathway can be activated to restore function to a hemidiaphragm paralysed by upper cervical (C2) spinal cord hemisection during a reflex known as the 'crossed phrenic phenomenon'. In addition, theophylline, a general adenosine A1 and A2 receptor antagonist, can activate the latent pathway by acting centrally through antagonism at adenosine receptors. 2. The present study was designed to assess the relative contributions of adenosine A1 and A2 receptors in inducing functional recovery in our model of spinal cord injury. Specific adenosine A1 and A2 agonists and antagonists were used in an electrophysiological study. 3. Our results demonstrate that, in hemisected rats, systemic administration of the adenosine A1 receptor-specific antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) restores, in a dose-dependent manner, phrenic nerve respiratory related output that is lost following hemisection. Furthermore, DPCPX augments respiratory activity in non-injured animals. The A2 receptor agonist CGS-21680 mediates its effects by predominantly acting on peripheral rather than central nervous system (CNS) receptors. CGS-21680 modulates respiratory related phrenic nerve activity in non-injured animals by enhancing tonic activity, but does not induce recovery of phrenic nerve activity in hemisected animals in the majority of cases. When CGS-21680 was administered prior to DPCPX in hemisected rats, the magnitude of recovery of respiratory function was significantly greater than that elicited by DPCPX alone. However, when the A2 receptor agonist was administered after DPCPX, the magnitude of recovery was virtually unchanged, whereas activity in the right phrenic nerve was significantly enhanced. The A1 receptor agonist N6-cyclohexyladenosine depressed respiratory activity in non-injured, as well as hemisected, rats. The A2 receptor antagonist 3,7-dimethyl-1-propargylxanthine did not affect respiratory activity. 4. We conclude that while antagonism at central adenosine A1 receptors mediates functional restitution in hemisected animals, activation of A2 receptors located outside of the CNS subserves the A1 receptor-mediated respiratory recovery.

Chronic cervical spinal sensory denervation reveals ineffective spinal pathways to phrenic motoneurons in the rat

We hypothesized that pretreatment with chronic cervical dorsal rhizotomy (CDR; C(3)-C(6)) would reveal ineffective crossed spinal pathways to phrenic motoneurons. Anesthetized CDR (1 week post-rhizotomy) and control rats were spinally hemisected at C(2), and phrenic potentials were evoked by stimulating the ventrolateral funiculus contralateral and rostral to hemisection. Phrenic potentials contralateral to the stimulating electrode were evoked at lower stimulus currents (CDR=640 +/- 46 microA; control=900 +/- 50 microA; P<0.05) and potential amplitude was significantly greater in CDR versus control rats (P<0.05). The serotonin receptor antagonist methysergide (4 mg/kg, i.v.) had no effect on the crossed phrenic potential amplitude (91+/-17% of control at 800 microA; P>0.05). Thus, CDR enhances crossed phrenic pathways but serotonin receptor activation is not necessary to maintain this effect.

Cervical spinal cord injury alters the pattern of breathing in anesthetized rats

The mechanisms by which chronic cervical spinal cord injury alters respiratory function and plasticity are not well understood. We speculated that spinal hemisection at C(2) would alter the respiratory pattern controlled by vagal mechanisms. Expired volume (V(E)) and respiratory rate (RR) were measured in anesthetized control and C(2)-hemisected rats at 1 and 2 mo postinjury. C(2) hemisection altered the pattern of breathing at both postinjury time intervals. Injured rats utilized a higher RR and lower V(E) to maintain the same minute ventilation as control rats. After bilateral vagotomy, the pattern of breathing in injured rats was not different from controls. The frequency of augmented breaths was higher in injured rats at 2 mo postinjury before vagotomy; however, the V(E) of augmented breaths was not different between groups. In conclusion, C(2) hemisection alters the pattern of breathing at 1 and 2 mo postinjury via vagal mechanisms.

Activation of a latent respiratory motor pathway by stimulation of neurons in the medullary chemoreceptor area of the rat

Previous studies have demonstrated that during respiratory stress (hypercapnia and hypoxia), a latent crossed respiratory pathway can be activated to produce hemidiaphragm recovery following an ipsilateral C2 spinal cord hemisection. The present study investigates the effects of ventral medullary chemoreceptor area stimulation by microinjection of (1S,3R)-aminocyclopentanedicarboxylic acid (ACPD), a glutamate metabotropic receptor agonist, on activating the latent pathway following left C2 spinal cord hemisection in rats in which end-tidal CO2 was maintained at a constant level. Experiments were conducted on anesthetized, vagotomized, paralyzed, and artificially ventilated rats in which phrenic nerve activity was recorded bilaterally. Before drug injection, the phrenic nerve contralateral to hemisection showed vigorous respiratory-related activity, but the phrenic nerve ipsilateral to hemisection showed no discernible respiratory-related activity. ACPD (1-100 nl, 1 mM) was injected directly into the region of the retrotrapezoid nucleus (RTN), a known medullary chemoreceptor area. Microinjection of ACPD into the right RTN increased respiratory-related activity in the right phrenic nerve (contralateral to hemisection). ACPD (>5 nl, 1 mM) microinjection also significantly induced respiratory recovery in the phrenic nerve ipsilateral to hemisection in a dose-dependent manner. The present study indicates that respiratory recovery can be achieved by stimulation of respiratory circuitry without increasing CO2 levels.

Alkylxanthine-induced recovery of respiratory function following cervical spinal cord injury in adult rats

Previous investigations from our laboratory have demonstrated qualitatively that a latent respiratory pathway can be activated by systemic theophylline administration to restore function to a hemidiaphragm paralyzed by an upper (C2) cervical spinal cord hemisection in adult rats. The present study seeks to extend the previous investigations by contrasting and quantitating the actions of theophylline, 8-phenyltheophylline, enprofylline, and 8(p-Sulfophenyl)theophylline in restoring function 24 h after hemidiaphragm paralysis. The alkylxanthines were selected based on their diverse pharmacologic profiles to elucidate the mechanisms that underlie functional recovery after spinal cord injury. To quantitatively assess the magnitude of recovery, electrophysiological experiments were conducted on pancuronium-paralyzed, hemisected animals under standardized recording conditions. The total absence of respiratory-related activity in the phrenic nerve ipsilateral to the hemisection and paralyzed hemidiaphragm was used as the index of a functionally complete hemisection. Thereafter, drug-induced recovered activity in the phrenic nerve ipsilateral to hemisection was quantified and expressed either as a percentage of contralateral phrenic nerve activity in the same animal prior to drug administration or as a percentage of predrug activity in the homolateral nerve in noninjured animals. With either approach, theophylline (5-15 mg/kg) and 8-phenyltheophylline (5-10 mg/kg) dose-dependently induced respiratory-related recovered activity. Enprofylline, a potent bronchodilator, and 8(p-Sulfophenyl)theophylline, an adenosine receptor antagonist with limited access to the central nervous system, were ineffective. Maximal recovery was attained with theophylline (15 mg/kg) and 8-phenyltheophylline (10 mg/kg). At these doses, theophylline and 8-phenyltheophylline induced recovery that was 70.0 +/- 2.5 and 69.3 +/- 4.1% of predrug contralateral nerve activity respectively. When expressed as a percentage of activity in the homolateral nerve in noninjured animals, the magnitude changed to 32.9 +/- 4.9 and 35.7 +/- 6.9%, respectively. Involvement of adenosine receptors in the alkylxanthine-induced actions was confirmed in experiments with the adenosine analog, N6 (l-2-phenylisopropyl) adenosine (L-PIA). It is concluded that central adenosine receptor-mediated mechanisms are implicated in the recovery of respiratory-related activity after spinal cord injury. Furthermore, our results suggest a potential for a new therapeutic approach in the rehabilitation of spinal cord patients with respiratory deficits.

Plasticity of cardiorespiratory neural processing: classification and computational functions

Neural plasticity, or malleability of neuronal structure and function, is an important attribute of the mammalian forebrain and is generally thought to be a kernel of biological intelligence. In this review, we examine some reported manifestations of neural plasticity in the cardiorespiratory system and classify them into four functional categories, integral; differential; memory; and statistical-type plasticity. At the cellular and systems level the myriad forms of cardiorespiratory plasticity display emergent and self-organization properties, use- and disuse-dependent and pairing-specific properties, short-term and long-term potentiation or depression, as well as redundancy in series or parallel structures, convergent pathways or backup and fail-safe surrogate pathways. At the behavioral level, the cardiorespiratory system demonstrates the capability of associative and nonassociative learning, classical and operant conditioning as well as short-term and long-term memory. The remarkable similarity and consistency of the various types of plasticity exhibited at all levels of organization suggest that neural plasticity is integral to cardiorespiratory control and may subserve important physiological functions.

Effects of serotonin inhibition on neuronal and astrocyte plasticity in the phrenic nucleus 4 h following C2 spinal cord hemisection

C2 spinal cord hemisection results in synaptic and astroglial changes in the phrenic nucleus which have been associated with the recovery of the ipsilateral hemidiaphragm during expression of the crossed phrenic phenomenon. As part of our ongoing analysis of the neurotransmitters involved, the present study investigated the effects of systemic administration of para-chlorophenylalanine (p-CPA), a serotonin (5-HT) synthesis inhibitor, on plasticity in the rat phrenic nucleus 4 h following C2 hemisection. Hemisected control rats demonstrated typical morphological changes in the ipsilateral phrenic nucleus including: (1) an increased number and length of synaptic active zones and (2) an increased number and length of dendrodendritic membrane appositions. p-CPA treatment 3 days prior to hemisection reduced 5-HT levels and resulted in an attenuation of these changes in the ipsilateral phrenic nucleus 4 h following hemisection compared to hemisected controls. In addition, p-CPA treatment attenuated injury-induced alterations in immunohistochemical staining of glial fibrillary acidic protein (GFAP), although Western blot analysis demonstrated that overall levels of GFAP did not differ significantly between groups. The results suggest that inhibition of 5-HT synthesis by p-CPA attenuates hemisection-induced plasticity in the phrenic nucleus 4 h following an ipsilateral C2 hemisection.

The effects of mitotic inhibition on the spinal cord response to the superimposed injuries of spinal cord hemisection and peripheral axotomy

The present study was carried out to test the hypothesis that dividing microglia are responsible for the depression of crossed phrenic nerve activity documented at 2 weeks postphrenicotomy in an injury model which superimposes the effects of spinal cord injury on peripheral axotomy. Crossed phenic nerve activity is defined as the respiratory activity recorded from the phrenic nerve during the crossed phrenic phenomenon (CPP) which is a respiratory reflex induced by respiratory stress following an ispsilateral spinal cord hemisection. Young adult female Sprague-Dawley rats were subjected to left intrathoracic phrenicotomies. Cytarabine (Cyt-A, a powerful antimitotic drug) or saline-filled miniosmotic pumps were then implanted into the cisterna magna and 2 weeks were allowed to pass at which time the CPP was induced by a left C2 spinal cord hemisection and transection of the contralateral phrenic nerve. Control studies including bromodeoxyuridine labeling of mitotic cells and a triple immunofluorescent protocol were carried out to verify that microglial cells were the primary cell type undergoing mitosis in the current injury model and that Cyt-A completely inhibited cellular proliferation. Quantitative electrophysiological analysis of crossed phrenic nerve activity showed that there is a statistically significant depression of activity at 2 weeks postphrenicotomy when animals were infused with saline compared to controls. Crossed phrenic nerve activity levels were not significantly different, however, from control levels when 2-week postphrenicotomized rats were infused with Cyt-A. Immunofluorescent studies showed that the majority of cells dividing in response to phrenicotomy were microglia. Furthermore, there were no astrocytes seen dividing at any time. From the results, we conclude that activated microglial cells may be responsible for the depression in crossed phrenic activity normally seen 2 weeks postphrenicotomy. Further, the activation of microglia may be related to the astrocytic response to injury. The activated microglial cell may be acting as a coordinator of various aspects of the injury response. Alternatively, the activation of microglia may be a necessary step in the cascade of multiple events that take place in the spinal cord after injury.

The use of single phrenic axon recordings to assess diaphragm recovery after cervical spinal cord injury

Electrophysiological recordings taken from the whole phrenic nerve have been utilized previously to describe the gradual increase in functional recovery of a hemidiaphragm paralyzed by ipsilateral C2 hemisection during the crossed phrenic phenomenon (CPP). Although the increase in activity has been temporally correlated with hemisection-induced morphological alterations of the phrenic nucleus, suggesting an association of the increased activity with the morphological alterations, whole phrenic nerve recordings during the CPP can provide only limited information. The purpose of the present study, therefore, was to use phrenic single-axon recording techniques to better understand the mechanisms underlying the recovery of respiratory activity during the expression of the CPP. Recordings from the whole phrenic nerve on the right side and from small fascicles of the phrenic nerve that show only the activity of single phrenic axons (units) on the left side were made in the neck before left spinal hemisection and during the CPP. The results indicated that there were two types of units firing before and during the CPP: an early- and a late-firing unit based on the time of their firing onset in relation to whole phrenic nerve activity. Ten early units and 25 late units were identified according to the shape of their spikes before hemisection as well as during the CPP. In addition to these units, 20 new units were recruited during CPP activity. These new units were mainly of the late-onset type. The results also indicated that there was a significant increase in the frequency of firing of both early and late units. The results specifically indicate therefore that the increase in respiratory activity recorded previously in the whole phrenic nerve during the CPP is most likely due to: (i) an increase in firing frequency for both early- and late-firing units and (ii) a recruitment of predominantly late-firing units into the CPP response. These results are important in understanding more completely the mechanisms that can facilitate recovery of the diaphragm after spinal cord injury.

Theophylline-induced recovery in a hemidiaphragm paralyzed by hemisection in rats: contribution of adenosine receptors

Previously, we demonstrated that a single intravenous injection of theophylline can induce recovery in a hemidiaphragm paralyzed by cervical (C2) spinal cord hemisection for up to 3 h. The present study contrasts the actions of enprofylline and theophylline on inducing hemidiaphragmatic recovery after cervical spinal cord hemisection. Both drugs are methylxanthines; however, theophylline is an adenosine receptor antagonist while enprofylline is not. To further test the involvement of adenosine receptors, N6 (L-2-phenylisopropyl) adenosine (L-PIA), an analogue of adenosine was used in conjunction with theophylline. Following a left C2 spinal cord hemisection, animals were injected with either enprofylline (2.5-20 mg/kg) or theophylline (15 mg/kg) alone or in combination. Theophylline-injected animals demonstrated robust respiratory-related activity in the previously quiescent left phrenic nerve and hemidiaphragm. No recovery was observed in any of the enprofylline-injected rats. When enprofylline injection was followed later with theophylline, recovery occurred. Prior L-PIA administration blocked theophylline-induced recovery. When given after theophylline, L-PIA attenuated and then blocked the induced activity in both the nerve and hemidiaphragm ipsilateral to spinal cord hemisection. We conclude that adenosine receptor antagonism is implicated in hemidiaphragmatic recovery after hemisection and theophylline may be useful in the treatment of spinal cord injured patients with respiratory deficits.

Effects of chronic systemic theophylline injections on recovery of hemidiaphragmatic function after cervical spinal cord injury in adult rats

Based on a previous demonstration that acutely administered theophylline induces respiratory-related recovery in an animal model of spinal cord injury, the influence of chronically administered theophylline on maintaining recovery was assessed. The absence of respiratory-related activity in the left phrenic nerve and hemidiaphragm of rats subjected to an ipsilateral C2 spinal cord hemisection was confirmed electrophysiologically 24 h after injury. Theophylline was then injected i.p. for 3-30 consecutive days. Recovery of respiratory-related activity was observed in the majority (29 out of 32) of the experimental animals. We conclude that theophylline not only induces, but also maintains recovery for prolonged periods after cervical spinal cord injury.

Glial changes in the phrenic nucleus following superimposed cervical spinal cord hemisection and peripheral chronic phrenicotomy injuries in adult rats

The objective of the present study was to characterize the microglial and astroglial reaction in the phrenic nucleus following either an ipsilateral C2 spinal cord hemisection, a peripheral phrenicotomy, or a combination of the two injuries in the same adult rat. The present study used three different fluorescent markers and a confocal laser image analysis system to study glial cells and phrenic motoneurons at the light microscopic level. Young adult female rats were divided into one combined injury group (left phrenicotomy and left C2 spinal hemisection with periods of 1 to 4 weeks between injuries, N = 12) and three other groups consisting of noninjured animals (N = 3), animals that received C2 hemisection only (N = 3), and animals with phrenicotomy only (survival periods of 2 (N = 3) and 4 (N = 3) weeks after phrenicotomy). Fluorogold was injected into the diaphragm to label phrenic motoneurons in all animals. Microglia and astrocytes were labeled with Texas red and fluorescein, respectively, and were visualized simultaneously along with phrenic motoneurons. The results suggest that the microglial and astrocytic response in the superimposed injury model are similar to the glial reactions characteristically seen in a peripheral axotomy alone model. These reactions include proliferation and migration of microglial cells along the perineuronal surface (peaking at 2 weeks) and the hypertrophy of astrocytes (peaking at 4 weeks). In addition, the increase in astrocytic tissue, which is characteristically seen in response to axotomy alone, is significantly enhanced in the superimposed injury model. Also, there is a large and rapid increase in GFAP-positive astrocytes within 24 hours after hemisection alone. The information gained from the present study will aid in determining, predicting, and eventually manipulating central nervous system responses to multiple injuries with the objective of reestablishing function in the damaged CNS.

Morphological plasticity induced in the phrenic nucleus following cervical cold block of descending respiratory drive

Morphological plasticity occurs in the phrenic nucleus within hours following an ipsilateral C2 spinal cord hemisection. The plasticity has been associated with the unmasking of a latent respiratory pathway (the crossed phrenic pathway) which allows recovery of the hemidiaphragm paralyzed by the hemisection during a reflex known as the crossed phrenic phenomenon. This study tests if the plasticity is induced by the generalized effects of spinal cord trauma or the more specific effect of interrupting the main descending respiratory drive to phrenic motoneurons. Electron microscopic quantitative morphometric analysis of the phrenic nucleus neuropil was carried out on four Sprague-Dawley rats (200-250 g) sacrificed 4 h following unilateral reversible cold block of the descending bulbospinal respiratory drive at the second cervical segment of the spinal cord (C2). The data from four sham-operated control animals were compared with those of the experimental group. The following morphological alterations were documented in cold block animals compared to controls: (1) a significant increase in the number of multiple synapses (i.e., terminals with synaptic active zones contacting two or more postsynaptic profiles in the same plane of section), (2) a significant increase in the number of dendrodendritic appositions, and (3) a significant increase in the length of symmetric and asymmetric synaptic active zones. The above changes are similar to the changes induced in the phrenic nucleus following C2 hemisection. We conclude therefore, that injury to the spinal cord is not a requirement for this type of morphological plasticity in the phrenic nucleus, but rather the induced changes are activity-dependent and are likely caused by the interruption of the descending bulbospinal respiratory drive to the phrenic nucleus.

Ultrastructural quantitative analysis of glutamatergic and GABAergic synaptic terminals in the phrenic nucleus after spinal cord injury

Quantitative analysis of electron microscopic postembedding immunochemically stained material indicates that 48% of all terminals in the rat phrenic nucleus are glutamatergic and 33% are gamma-aminobutyric acid (GABA)ergic. Three distinct types of glutamatergic terminals were observed in the rat phrenic nucleus: terminals characterized by large, loosely arranged spherical synaptic vesicles (SI) or small, compact spherical synaptic vesicles (Ss) and elongated terminals containing spherical synaptic vesicles with neurofilaments (NFs). All three types of glutamatergic terminals display asymmetrical synaptic membrane densities with postsynaptic dense bodies being present in some of the S-type terminals. The GABAergic immunoreactive terminals in the phrenic nucleus most closely resemble F-type terminals. They are characterized by flattened or pleomorphic synaptic vesicles and symmetric synaptic membrane densities. Among the 48% glutamatergic terminals, 27% are SI, 65% are Ss, and 8% are NFs, respectively. Significantly fewer glutamate, GABA, and unlabeled terminals per unit area are present in the phrenic nucleus 30 days after a C2 spinal cord hemisection as compared to nonhemisected controls. The average number of active zones per terminal, however, is greater in the hemisection group (1.45 +/- 0.03) than in the control group (1.34 +/- 0.03), with the active zones in the glutamate terminals mainly accounting for this difference. Moreover, the length of the active zones in the glutamate terminals was significantly longer in the hemisection group (0.37 +/- 0.013 microns) as compared to the controls (0.24 +/- 0.008 microns). In addition, the mean length of synaptic active zones in GABAergic terminals was also found to be longer in the hemisection group (0.36 +/- 0.022 microns) as compared to controls (0.28 +/- 0.014 microns). Finally, there is also a significantly higher ratio of synaptic active zones to the total number of glutamate-labeled terminals after injury (1.73 +/- 0.08) as compared to controls (1.41 +/- 0.04). The number of double/multiple synapses, the percentages of Sl, Ss, and NFs-type terminals, and the percentages of synaptic active zones contacting either distal dendrites or proximal dendrites/somata do not change significantly 30 days after injury. These results are important for a more complete understanding of the synaptic plasticity that occurs in the phrenic nucleus after spinal cord injury and to show how the plasticity may relate to the unmasking of latent bulbospinal respiratory connections which restore function to the hemidiaphragm paralyzed by an ipsilateral spinal cord hemisection.

Phrenic responses to contralateral spinal stimulation in rats: effects of old age or chronic spinal hemisection

Serotonin reveals ineffective spinal pathways from the C2-lateral funiculus to contralateral phrenic motoneurons in young adult rats with acute spinal hemisection. We tested the hypothesis that old age (1.5-2 years) or chronic hemisection (3-5 days) strengthens these pre-existing crossed spinal pathways. There were no consistent differences between young adult rats with acute hemisection versus young adult rats with chronic hemisection or old rat with acute hemisection except that one long-latency phrenic excitation could not be elicited in old rats. The results indicate that neither old age nor chronic hemisection strengthens crossed-spinal pathways, but that old age may selectively diminish spinal pathways involved in the neural control of breathing.

Quantitative assessment of phrenic nerve functional recovery mediated by the crossed phrenic reflex at various time intervals after spinal cord injury

The present study was carried out to determine if augmentation of phrenic nerve activity during the crossed phrenic phenomenon temporally coincides with the morphological changes in the phrenic nucleus that we have observed in previous studies. This investigation consisted of two experiments in spinal cord hemisected young adult female Sprague-Dawley rats. Crossed phrenic activity was quantitatively assessed from the left phrenic nerve after bilateral vagotomy and sectioning of the right phrenic and accessory phrenic nerves. The first experiment involved serial recordings of crossed phrenic activity performed on each of 4 animals at hourly intervals ranging from 1 to 6 h after spinal cord hemisection. The second experiment consisted of single recordings from each of 24 animals at one of the following time intervals after hemisection: 1/2, 1, 2, 4, 12, and 24 h. Recording conditions were standardized at each recording session in both experiments by paralyzing the animals, regulating temperature and blood pressure, and controlling end tidal PCO2 with a volume ventilator. Crossed phrenic activity was induced by stopping the ventilator and quantitated by measuring the area under the integrated waveform of the largest respiratory burst. The results revealed a small, statistically insignificant increase in crossed phrenic activity at 1 h compared to the 30-min recordings. At 2 h there was a large, statistically significant increase in activity. Experiment one showed further increases from 3 to 6 h. The second experiment showed a smaller increase from 2 to 4 h and then maintained this level at 12 and 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Decussation of bulbospinal respiratory axons at the level of the phrenic nuclei in adult rats: a possible substrate for the crossed phrenic phenomenon

The axonal trajectories of inspiratory bulbospinal neurons were examined after deposition of the anterograde neuronal tracer phaseolus vulgaris leucoagglutinin (PHA-L) into the rostral ventral respiratory group in rats. At the level of the phrenic nucleus, PHA-L-labeled bulbospinal axons crossed the midline of the spinal cord in both the anterior gray and the anterior white commissure. These spinally decussating neurons provide a possible anatomical substrate for the respiratory reflex known as the crossed phrenic phenomenon.

Chronic hypoxia causes morphological alterations in astroglia in the phrenic nucleus of young adult rats

The present study was carried out to determine if chronic hypoxia in noninjured rats causes morphological alterations in the phrenic nucleus similar to those caused by spinal cord hemisection rostral to the nucleus. Normal rats were subjected to chronic hypoxia in an atmosphere chamber for 48 h. A Bioquant system was used to quantitatively analyze electron micrographs through the phrenic nuclei of chronically hypoxic and normal rats. The results indicated that chronic hypoxia caused astroglial processes to retract from between adjacent dendrites, leading to a significant increase in phrenic dendrodendritic membrane apposition. Unlike spinal cord injury, however, chronic hypoxia did not induce synaptogenesis in the phrenic nucleus. Specifically, the mean length of phrenic dendrodendritic appositions increased significantly from a normal value of 1.42 +/- 0.09 to 1.82 +/- 0.11 microns in hypoxic animals. Moreover, the mean number of dendrodendritic appositions in the normal phrenic nucleus (16.75 +/- 1.11) increased markedly (24.00 +/- 3.54) in the phrenic nucleus of hypoxic rats. There was no significant difference between the mean number of double (17.75 +/- 1.03) or multiple (20.00 +/- 1.47) synapses in normal rats as compared to the mean number of double (18.50 +/- 3.10) or multiple (20.25 +/- 2.84) synapses in the phrenic nucleus of hypoxic animals. From the present results, it is concluded that the rapid synaptogenesis in the phrenic nucleus following spinal cord hemisection is induced by the generalized effects of spinal cord injury and astroglial process retraction does not necessarily lead to synapse formation. The physiological significance of the astroglial movement, however, needs elucidation by additional experiments.

Neuronal and glial changes in the rat phrenic nucleus occurring within hours after spinal cord injury

The present study describes specific morphological changes in the normal ultrastructure of the rat phrenic nucleus which occur within 4 hours after an ipsilateral spinal cord hemisection rostral to the nucleus. Phrenic neurons were identified at EM levels by retrograde HRP labeling. Ultrastructural features of the phrenic nucleus in uninjured animals and at 4 hours and 1, 2, and 4 days after injury were qualitatively analyzed and then quantitated with a computerized morphometric system. Our results indicated that by 4 hours posthemisection, there was a significant increase in the number of double synapses. Furthermore, the number of double synapses remained significantly higher than normal at all the other posthemisection periods. A significant increase in the length of dendrodendritic membrane appositions was also noted as early as 4 hours posthemisection. The mean normal appositional length of 1.42 +/- 0.09 microns increased to 1.89 +/- 0.12 microns at 4 hours and further increased to 2.20 +/- 0.20 microns by 1 day posthemisection. The increase in the length of membrane appositions was most likely due to an active retraction of astroglial processes from their normal position in between the dendrites. Although there was an increase in the mean length of the dendrodendritic appositions, the mean percentage of the appositions (expressed as the total number of appositions divided by the total number of dendrites in each sample) was not increased significantly over normal values during the early posthemisection periods. By 2 and 4 days posthemisection, however, the percentage of dendrodendritic appositions increased to significantly higher values than normal. Normally, 4.68 +/- 0.69% of the dendrites in the phrenic nucleus were found to be in apposition, and this number increased significantly to 7.27 +/- 1.06% by 2 days and 7.46 +/- 0.79% by 4 days posthemisection. At these later posthemisection periods, the mean length of the appositions decreased to levels which were no longer significantly higher than normal. A distribution analysis of the length of each dendrodendritic apposition in both the normal and spinal hemisected rats showed that there were more dendrodendritic appositions in the phrenic nucleus at the later posthemisection periods. It also showed that their mean length was decreased because many of the new appositions were relatively short. The above neuronal and glial alterations of the phrenic nucleus have never before been described as a response to injury of the mammalian spinal cord. Furthermore, the possibility that the above changes could represent the morphological substrate for the unmasking of functionally ineffective synapses in ou