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

Intermittent Hypoxia Differentially Regulates Adenosine Receptors in Phrenic Motor Neurons with Spinal Cord Injury

Cervical spinal cord injury (cSCI) impairs neural drive to the respiratory muscles, causing life- threatening complications such as respiratory insufficiency and diminished airway protection. Repetitive "low dose" acute intermittent hypoxia (AIH) is a promising strategy to restore motor function in people with chronic SCI. Conversely, "high dose" chronic intermittent hypoxia (CIH; ∼8 h/night), such as experienced during sleep apnea, causes pathology. Sleep apnea, spinal ischemia, hypoxia and neuroinflammation associated with cSCI increase extracellular adenosine concentrations and activate spinal adenosine receptors which in turn constrains the functional benefits of therapeutic AIH. Adenosine 1 and 2A receptors (A₁, A_2A) compete to determine net cAMP signaling and likely the tAIH efficacy with chronic cSCI. Since cSCI and intermittent hypoxia may regulate adenosine receptor expression in phrenic motor neurons, we tested the hypotheses that: 1) daily AIH (28 days) downregulates A_2A and upregulates A₁ receptor expression; 2) CIH (28 days) upregulates A_2A and downregulates A₁ receptor expression; and 3) cSCI alters the impact of CIH on adenosine receptor expression. Daily AIH had no effect on either adenosine receptor in intact or injured rats. However, CIH exerted complex effects depending on injury status. Whereas CIH increased A₁ receptor expression in intact (not injured) rats, it increased A_2A receptor expression in spinally injured (not intact) rats. The differential impact of CIH reinforces the concept that the injured spinal cord behaves in distinct ways from intact spinal cords, and that these differences should be considered in the design of experiments and/or new treatments for chronic cSCI.

Phrenic motor neuron survival below cervical spinal cord hemisection

Cervical spinal cord injury (cSCI) severs bulbospinal projections to respiratory motor neurons, paralyzing respiratory muscles below the injury. C2 spinal hemisection (C2Hx) is a model of cSCI often used to study spontaneous and induced plasticity and breathing recovery post-injury. One key assumption is that C2Hx dennervates motor neurons below the injury, but does not affect their survival. However, a recent study reported substantial bilateral motor neuron death caudal to C2Hx. Since phrenic motor neuron (PMN) death following C2Hx would have profound implications for therapeutic strategies designed to target spared neural circuits, we tested the hypothesis that C2Hx minimally impacts PMN survival. Using improved retrograde tracing methods, we observed no loss of PMNs at 2- or 8-weeks post-C2Hx. We also observed no injury-related differences in ChAT or NeuN immunolabeling within labelled PMNs. Although we found no evidence of PMN loss following C2Hx, we cannot rule out neuronal loss in other motor pools. These findings address an essential prerequisite for studies that utilize C2Hx as a model to explore strategies for inducing plasticity and/or regeneration within the phrenic motor system, as they provide important insights into the viability of phrenic motor neurons as therapeutic targets after high cervical injury.

Intermittent hypoxia and respiratory recovery in pre-clinical rodent models of incomplete cervical spinal cord injury

Impaired respiratory function is a common and devastating consequence of cervical spinal cord injury. Accordingly, the development of safe and effective treatments to restore breathing function is critical. Acute intermittent hypoxia has emerged as a promising therapeutic strategy to treat respiratory insufficiency in individuals with spinal cord injury. Since the original report by Bach and Mitchell (1996) concerning long-term facilitation of phrenic motor output elicited by brief, episodic exposure to reduced oxygen, a series of studies in animal models have led to the realization that acute intermittent hypoxia may have tremendous potential for inducing neuroplasticity and functional recovery in the injured spinal cord. Advances in our understanding of the neurobiology of acute intermittent hypoxia have prompted us to begin to explore its effects in human clinical studies. Here, we review the basic neurobiology of the control of breathing and the pathophysiology and respiratory consequences of two common experimental models of incomplete cervical spinal cord injury (i.e., high cervical hemisection and mid-cervical contusion). We then discuss the impact of acute intermittent hypoxia on respiratory motor function in these models: work that has laid the foundation for translation of this promising therapeutic strategy to clinical populations. Lastly, we examine the limitations of these animal models and intermittent hypoxia and discuss how future work in animal models may further advance the translation and therapeutic efficacy of this treatment.

Oxygen transport kinetics underpin rapid and robust diaphragm recovery following chronic spinal cord injury

Key points

Spinal treatment can restore diaphragm function in all animals 1 month following C2 hemisection induced paralysis. Greater recovery occurs the longer after injury the treatment is applied.

Through advanced assessment of muscle mechanics, innovative histology and oxygen tension modelling, we have comprehensively characterized in vivo diaphragm function and phenotype.

Muscle work loops reveal a significant deficit in diaphragm functional properties following chronic injury and paralysis, which are normalized following restored muscle activity caused by plasticity‐induced spinal reconnection.

Injury causes global and local alterations in diaphragm muscle vascular supply, limiting oxygen diffusion and disturbing function. Restoration of muscle activity reverses these alterations, restoring oxygen supply to the tissue and enabling recovery of muscle functional properties.

There remain metabolic deficits following restoration of diaphragm activity, probably explaining only partial functional recovery. We hypothesize that these deficits need to be resolved to restore complete respiratory motor function.

Abstract

Months after spinal cord injury (SCI), respiratory deficits remain the primary cause of morbidity and mortality for patients. It is possible to induce partial respiratory motor functional recovery in chronic SCI following 2 weeks of spinal neuroplasticity. However, the peripheral mechanisms underpinning this recovery are largely unknown, limiting development of new clinical treatments with potential for complete functional restoration. Utilizing a rat hemisection model, diaphragm function and paralysis was assessed and recovered at chronic time points following trauma through chondroitinase ABC induced neuroplasticity. We simulated the diaphragm's in vivo cyclical length change and activity patterns using the work loop technique at the same time as assessing global and local measures of the muscles histology to quantify changes in muscle phenotype, microvascular composition, and oxidative capacity following injury and recovery. These data were fed into a physiologically informed model of tissue oxygen transport. We demonstrate that hemidiaphragm paralysis causes muscle fibre hypertrophy, maintaining global oxygen supply, although it alters isolated muscle kinetics, limiting respiratory function. Treatment induced recovery of respiratory activity normalized these effects, increasing oxygen supply, restoring optimal diaphragm functional properties. However, metabolic demands of the diaphragm were significantly reduced following both injury and recovery, potentially limiting restoration of normal muscle performance. The mechanism of rapid respiratory muscle recovery following spinal trauma occurs through oxygen transport, metabolic demand and functional dynamics of striated muscle. Overall, these data support a systems‐wide approach to the treatment of SCI, and identify new targets to mediate complete respiratory recovery.

Spinal treatment can restore diaphragm function in all animals 1 month following C2 hemisection induced paralysis. Greater recovery occurs the longer after injury the treatment is applied.

Through advanced assessment of muscle mechanics, innovative histology and oxygen tension modelling, we have comprehensively characterized in vivo diaphragm function and phenotype.

Muscle work loops reveal a significant deficit in diaphragm functional properties following chronic injury and paralysis, which are normalized following restored muscle activity caused by plasticity‐induced spinal reconnection.

Injury causes global and local alterations in diaphragm muscle vascular supply, limiting oxygen diffusion and disturbing function. Restoration of muscle activity reverses these alterations, restoring oxygen supply to the tissue and enabling recovery of muscle functional properties.

There remain metabolic deficits following restoration of diaphragm activity, probably explaining only partial functional recovery. We hypothesize that these deficits need to be resolved to restore complete respiratory motor function.

Ecto-5'-nucleotidase (CD73) attenuates inflammation after spinal cord injury by promoting macrophages/microglia M2 polarization in mice

Background

Immune activation, specifically activation of macrophages and resident microglia, leading to inflammation is a key component in the progression of spinal cord injury (SCI). Macrophages/microglia exist in two states—the classically activated M1 phenotype that confers pro-inflammatory effects or the alternatively activated M2 phenotype that confers anti-inflammatory effects. Ecto-5′-nucleotidase (CD73) is an immunosuppressive molecule intricately involved in adaptive and innate immune responses and is able to dephosphorylate AMP to adenosine. However, it is not known if CD73 is able to modulate the macrophages/microglia transformation between the M1 and M2 phenotypes.

Methods

We used gene-deficient mice to determine the role of CD73 in macrophages/microglia polarization post-SCI in vivo. We used small interference RNA (siRNA) or pcDNA3.1 to inhibit or overexpress CD73 in BV2 cells to verify anterior discovery in vitro. A combination of molecular and histological methods was used to detect the macrophages/microglia polarization and explore the mechanism both in vivo and in vitro.

Results

We found that SCI induced the upregulation of CD73 expression. CD73 deficient mice were noted to demonstrate overwhelming immune responses, few anti-inflammatory phenotype macrophages/microglia, and had a poorer locomotor recovery in comparison to wild-type mice that were also inflicted with SCI. In vitro studies found that CD73 suppression inhibited the expression of characteristic microglial anti-inflammatory polarization markers in BV2 cells, while the converse was noted in CD73 overexpression. Subsequent experiments confirmed that CD73 promoted microglia alternative activation by stimulating p38 MAPK.

Conclusion

We were able to conclude that CD73 imparts neuroprotective effects by mediating macrophages/microglia polarization. These findings allow for better understanding of the modulatory factors involved in triggering the change in macrophages/microglia phenotypes, therefore uncovering additional molecules and pathways that may be targeted in the innovation of novel SCI therapies.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1183-8) contains supplementary material, which is available to authorized users.

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.

Ipsilateral inspiratory intercostal muscle activity after C2 spinal cord hemisection in rats

BACKGROUND

Upper cervical spinal cord hemisection causes paralysis of the ipsilateral hemidiaphragm; however, the effect of C2 hemisection on the function of the intercostal muscles is not clear. We hypothesized that C2 hemisection would eliminate inspiratory intercostal activity ipsilateral to the injury and that some activity would return in a time-dependent manner.

METHODS

Female Sprague Dawley rats were anesthetized with urethane and inspiratory intercostal electromyogram (EMG) activity was recorded in control rats, acutely injured C2 hemisected rats, and at 1 and 16 weeks post C2 hemisection.

RESULTS

Bilateral recordings of intercostal EMG activity showed that inspiratory activity was reduced immediately after injury and increased over time. EMG activity was observed first in rostral spaces followed by recovery occurring in caudal spaces. Theophylline increased respiratory drive and increased intercostal activity, inducing activity that was previously absent.

CONCLUSION

These results suggest that there are crossed, initially latent, respiratory connections to neurons innervating the intercostal muscles similar to those innervating phrenic motor neurons.

Theophylline regulates inflammatory and neurotrophic factor signals in functional recovery after C2-hemisection in adult rats

Recovery of respiratory activity in an upper cervical hemisection model (C2H) of spinal cord injury (SCI) can be induced by systemic theophylline administration 24-48 h after injury. The objectives in the present study are (1) to identify pro-inflammatory and neurotrophic factors expressed after C2H and (2) molecular signals involved in functional recovery. Four groups of adult female rats classified as (i) sham (SH) controls, (ii) subjected to a left C2 hemisection (C2H) only, (iii) C2H rats administered theophylline for 3 consecutive days 2 days after C2H (C2H-T day 5) and (iv) C2H rats treated with theophylline for 3 consecutive days 2 days after C2H and then weaned for 12 days (C2H-T day 17) prior to assessment of respiratory function and molecular analysis were employed. Corresponding sham controls, C2H untreated (vehicle only controls) and C2H treated (theophylline) rats were sacrificed, C3-C6 spinal cord segments quickly dissected and left (ipsilateral) hemi spinal cord and right (contralateral) hemi spinal cord were separately harvested 2 days post surgery. Sham operated and C2H untreated-controls corresponding to C2H-T day 5 and C2H-T day 17 rats, respectively, were prepared similarly. Messenger RNA levels for pro-inflammatory genes (TXNIP, IL-1β, TNF-α and iNOS) and neurotrophic and survival factors (BDNF, GDNF, and Bcl2) were analyzed by real time quantitative PCR. Gene expression pattern was unaltered in SH rats. TXNIP, iNOS, BDNF, GDNF and Bcl2 mRNA levels were significantly increased in the ipsilateral hemi spinal cord in C2H rats. BDNF, GDNF and Bcl2 levels remained elevated in the ipsilateral hemi spinal cord in C2H-T day 5 rats. In this same group, there was further enhancement in TXNIP and IL-1β while iNOS returned to basal levels. Theophylline increased DNA binding activity of transcription factors - cyclic AMP responsive element (CRE) binding protein (CREB) and pro-inflammatory NF-κB. Messenger RNA levels for all genes returned to basal levels in C2H-T day 17 rats. However, BDNF mRNA levels remained significantly elevated after weaning from the drug. Our results suggest that enhanced resolution of early inflammatory processes and expression of pro-survival factors may underlie theophylline-induced respiratory recovery. The results identify potential targets for gene and drug therapies.

Spinal cord injury sequelae alter drug pharmacokinetics: an overview

STUDY DESIGN

Literature review.

OBJECTIVES

Critical review of the literature published on the physiological alterations caused by spinal cord injury (SCI) and their effect on the pharmacokinetic parameters of commonly employed drugs. The review introduces the most recent treatment protocols of a variety of drugs, enabling the modern clinician to apply efficacious and cost-effective solutions to the pharmacological treatment of SCI patients.

METHODS

Studies published in international indexed journals up to January 2011 were selected from the PubMed database.

RESULTS

The review evaluated the sequelae of SCI and their effect on pharmacological processes. The results demonstrated that these alterations affected the pharmacokinetics of drugs commonly administered to SCI patients, such as antibiotics, muscle relaxants, immunosuppressants and analgesics.

CONCLUSION

There are multiple etiologies to SCI and patients present with varying degrees of impairment. Factors such as level of injury and completeness of the injury create a very heterogeneous population within the SCI community. The heterogeneity of this population creates a problem when trying to standardize pharmacokinetic (PK) parameters. It is because of this that specific physiological alterations must be linked to changes in PK and be identified within the clinical setting. This relationship between physiology and PK enables the clinician to be alert for possible pharmacological complications in individual patients based on their clinical manifestations. Future research should aim to develop rigorous therapeutic guidelines tailored to the diverse manifestations of SCI so as to provide effective, affordable and safe pharmacotherapy.

Theophylline treatment improves mitochondrial function after upper cervical spinal cord hemisection

The importance of mitochondria in spinal cord injury has mainly been attributed to their participation in apoptosis at the site of injury. But another aspect of mitochondrial function is the generation of more than 90% of cellular energy in the form of ATP, mediated by the oxidative phosphorylation (OxPhos) process. Cytochrome c oxidase (CcO) is a central OxPhos component and changes in its activity reflect changes in energy demand. A recent study suggests that respiratory muscle function in chronic obstructive pulmonary disease (COPD) patients is compromised via alterations in mitochondrial function. In an animal model of cervical spinal cord hemisection (C2HS) respiratory dysfunction, we have shown that theophylline improves respiratory function. In the present study, we tested the hypothesis that theophylline improves respiratory function at the cellular level via improved mitochondrial function in the C2HS model. We demonstrate that CcO activity was significantly (33%) increased in the spinal cord adjacent to the site of injury (C3-C5), and that administration of theophylline (20mg/kg 3x daily orally) after C2HS leads to an even more pronounced increase in CcO activity of 62% compared to sham-operated animals. These results are paralleled by a significant increase in cellular ATP levels (51% in the hemidiaphragm ipsilateral to the hemisection). We conclude that C2HS increases energy demand and activates mitochondrial respiration, and that theophylline treatment improves energy levels through activation of the mitochondrial OxPhos process to provide energy for tissue repair and functional recovery after paralysis in the C2HS model.

Shedding light on restoring respiratory function after spinal cord injury

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

Recovery of respiratory activity after C2 hemisection (C2HS): involvement of adenosinergic mechanisms

Consequences of spinal cord injury (SCI) depend on the level and extent of injury. Cervical SCI often results in a compromised respiratory system. Primary treatment of SCI patients with respiratory insufficiency continues to be with mechanical ventilatory support. In an animal model of SCI, an upper cervical spinal cord hemisection paralyzes the hemidiaphragm ipsilateral to the side of injury. However, a latent respiratory motor pathway can be activated to restore respiratory function after injury. In this review, restoration of respiratory activity following systemic administration of theophylline, a respiratory stimulant will be discussed. Pharmacologically, theophylline is a non-specific adenosine receptor antagonist, a phosphodiesterase inhibitor and a bronchodilator. It has been used in the treatment of asthma and other respiratory-related diseases such as chronic obstructive pulmonary disease (COPD) and in treatment of apnea in premature infants. However, the clinical use of theophylline to improve respiration in SCI patients with respiratory deficits is a more recent approach. This review will focus on the use of theophylline to restore respiratory activity in an animal model of SCI. In this model, a C2 hemisection (C2HS) interrupts the major descending respiratory pathways and paralyzes the ipsilateral hemidiaphragm. The review also highlights involvement of central and peripheral adenosine receptors in functional restitution. Biochemical binding assays that highlight changes in adenosine receptors after chronic theophylline administration are discussed as they pertain to understanding adenosine receptor-mediation in functional recovery. Finally, the clinical application of theophylline in SCI patients with respiratory deficits in particular is discussed.

Intermittent hypoxia induces functional recovery following cervical spinal injury

Respiratory-related complications are the leading cause of death in spinal cord injury (SCI) patients. Few effective SCI treatments are available after therapeutic interventions are performed in the period shortly after injury (e.g. spine stabilization and prevention of further spinal damage). In this review we explore the capacity to harness endogenous spinal plasticity induced by intermittent hypoxia to optimize function of surviving (spared) neural pathways associated with breathing. Two primary questions are addressed: (1) does intermittent hypoxia induce plasticity in spinal synaptic pathways to respiratory motor neurons following experimental SCI? and (2) can this plasticity improve respiratory function? In normal rats, intermittent hypoxia induces serotonin-dependent plasticity in spinal pathways to respiratory motor neurons. Early experiments suggest that intermittent hypoxia also enhances respiratory motor output in experimental models of cervical SCI (cervical hemisection) and that the capacity to induce functional recovery is greater with longer durations post-injury. Available evidence suggests that intermittent hypoxia-induced spinal plasticity has considerable therapeutic potential to treat respiratory insufficiency following chronic cervical spinal injury.

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 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.

Spinal NMDA receptor activation is necessary for de novo, but not the maintenance of, A2a receptor-mediated phrenic motor facilitation

Adenosine 2a (A2a) receptor agonists elicit persistent increases in phrenic nerve activity by transactivating the neurotrophin receptor, TrkB, near phrenic motoneurons. Our working model proposes that A2a receptor-mediated TrkB receptor activation strengthens glutamatergic synapses onto phrenic motoneurons. Activation of glutamate N-methyl d-aspartate (NMDA) receptors has been implicated in other models of phrenic motor plasticity. Thus we hypothesized that NMDA receptor activation also would contribute to A2a receptor-mediated phrenic motor facilitation. Adult male Sprague-Dawley rats were anesthetized with urethane, mechanically ventilated, neuromuscularly paralyzed, and bilaterally vagotomized. The A2a receptor agonist CGS-21680 and the NMDA receptor-channel blocker MK-801 were administered intrathecally over the C4 spinal segment. Phrenic nerve activity was recorded before, during, and after drug administration. MK-801 (concentration range 0.1, 1.0, 10.0, and 100 microM) was administered 30 min before CGS-21680 (50 microM). MK-801 dose-dependently blocked A2a receptor-mediated phrenic motor facilitation. When administered at 60 min post-CGS-21680, MK-801 prevented further increases in phrenic nerve activity compared with the CGS-21680 alone (CGS-21680 alone at 120 min: 114 +/- 19%; CGS-21680 and MK-801 at 60 min post-CGS-21680: 61 +/- 11%, above baseline, P < 0.05) but did not return phrenic motor output to baseline values. Our data suggest that NMDA receptor activation is necessary for de novo A2a receptor-mediated phrenic motor facilitation and that the maintenance of preexisting phrenic motor facilitation does not involve NMDA receptor-dependent mechanisms.

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.

Spinal activation of the cAMP-PKA pathway induces respiratory motor recovery following high cervical spinal cord injury

The present study investigated the involvement of the adenosine 3'5'-cyclic monophosphate-dependent protein kinase A (cAMP-PKA) pathway in the activation of the crossed-phrenic pathways after left C2 spinal cord hemisection. Experiments were conducted on left C2 spinal cord hemisected, anesthetized, vagotomized, pancuronium paralyzed, and artificially ventilated male Sprague-Dawley rats. One week post-injury, the ipsilateral phrenic nerve exhibited no respiratory-related activity indicating a functionally complete hemisection. Intrathecal spinal cord administration of the cAMP analog, 8-Br-cAMP at the level of the phrenic nucleus resulted in an enhancement of contralateral phrenic nerve output and a restoration of respiratory-related activity in the phrenic nerve ipsilateral to the hemisection. Furthermore, pre-treatment with Rp-8-Br-cAMP, a PKA inhibitor, abolished the effects of 8-Br-cAMP. These results suggest that PKA activation is necessary for the cAMP-mediated respiratory recovery following high cervical spinal cord injury and that activation of intracellular signaling cascades may represent an important strategy for improving respiratory function after spinal cord injury.

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.

Administration of phosphodiesterase inhibitors and an adenosine A1 receptor antagonist induces phrenic nerve recovery in high cervical spinal cord injured rats

High cervical spinal cord hemisection interrupts the descending respiratory drive from the medulla to the ipsilateral phrenic motoneurons, consequently leading to the paralysis of the ipsilateral hemidiaphragm. Previous studies have shown that chronic oral administration of theophylline, a phosphodiesterase inhibitor and an adenosine receptor antagonist, can restore function to the quiescent phrenic nerve and hemidiaphragm ipsilateral to hemisection. Both of these actions of theophylline result in an increase in 3'-5'-cyclic adenosine monophosphate (cAMP). Furthermore, the chronic theophylline-mediated respiratory recovery persists long after the animals have been weaned from the drug. To date, the precise cellular mechanisms underlying the recovery induced by theophylline are still not known. Since theophylline has two modes of action, in the present study we tested whether chronic administration of pentoxifylline, a non-selective phosphodiesterase inhibitor, rolipram, a phosphodiesterase-4 specific inhibitor, and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A1 receptor antagonist, would induce recovery similar to that induced by theophylline in male Sprague-Dawley rats following a left C2 spinal cord lesion. Recovery of left phrenic nerve activity was assessed at 5 or 10 days after the last drug administrations to assess the persistent nature of the recovery. Pentoxifylline, rolipram and DPCPX, all capable of modulating 3',5'-cyclic monophosphate (cAMP) levels, brought about long-term respiratory recovery in the phrenic nerve ipsilateral to the left C2 lesion at 5 and 10 days after the last drug administration. Therefore, these results suggest that compounds capable of regulating cAMP levels may be therapeutically useful in promoting functional recovery following spinal cord injury.

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.

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.

The Use of Small-Dose Theophylline for the Treatment of Bradycardia in Patients with Spinal Cord Injury

Small-dose theophylline can be a useful alternative in the treatment of bradycardia in patients with spinal cord injury. In this series of three cases, I describe its successful use for this indication. In one case, initiation of theophylline treatment also increased respiratory drive and minute ventilation. A possible mechanism is that theophylline increases the force of contraction of diaphragmatic muscles because of enhancement of calcium uptake through an adenosine-mediated channel. The antibradycardiac effect is probably mediated through one or more different molecular mechanisms, which seem to be mediated by inhibition of phosphodiesterase 3.

Spontaneous crossed phrenic activity in the neonatal respiratory network

Hemisection of the cervical spinal cord causes paralysis of the ipsilateral hemidiaphragm in adult rats. Activation of a latent crossed phrenic motor pathway can restore diaphragmatic function, although structural changes take place before the pathway can be activated. Since mechanisms are employed to eliminate non-functional projections during development, we predicted that this latent neural pathway might be active during development. Therefore, we examined the effect of spinal hemisection (C2) on respiratory-like activity bilaterally using the brainstem--spinal cord preparation from neonatal rats (0-4 days). Spontaneous crossed phrenic activity (respiratory-like activity recorded from the ipsilateral C4 or C5 ventral roots following C2 hemisection) was observed in an age-dependent manner; younger preparations exhibited more than older preparations. Increasing drive (increasing [K+] or superfusion of theophylline) either increased or induced crossed phrenic activity. Hemisection caused no change in the frequency, the burst area, duration or peak amplitude contralateral to hemisection. Unlike adult rats, this study shows that crossed phrenic activity is present in the in vitro respiratory network of neonatal rats suggesting that a crossed neural pathway may be functionally active in neonates.

Response of NADPH-Diaphorase-Exhibiting Neurons in the Medullar Reticular Formation to High Spinal Cord Injury

1. The effect of hemisection of the cervical spinal cord on NADPH-diaphorase staining in the reticular nuclei of the rabbit medulla was investigated using histochemical technique. 2. A quantitative assessment of somal and neuropil NADPH-diaphorase staining was made by an image analyzer in a selected area of each reticular nucleus of the rabbit medulla. 3. On the 7th postsurgery day, the highest up-regulation of somatic NADPH-diaphorase staining was observed in regions regulating cardiorespiratory processes; however, the highest increase of neuropil NADPH-diaphorase staining was found in the reticular nuclei modulating the tonus of postural muscles. 4. The degeneration of non-NADPH-diaphorase-stained neurons was detected throughout the reticular formation of the medulla, but the extent of neuronal death did not correlate with the up-regulation of the NADPH-diaphorase staining in the reticular nuclei of the medulla. 5. The findings provide evidence that NADPH-diaphorase-exhibiting neurons are refractory to the hemisection of the cervical spinal cord and that the neuronal up-regulation of NADPH-diaphorase at the medullar level is probably not a causative factor leading to the death of the reticulospinal neurons.

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.

Theophylline-induced respiratory recovery following cervical spinal cord hemisection is augmented by serotonin 2 receptor stimulation

Cervical spinal cord hemisection leads to a disruption of bulbospinal innervation of phrenic motoneurons resulting in paralysis of the ipsilateral hemidiaphragm. We have previously demonstrated separate therapeutic roles for theophylline, and more recently serotonin (5-HT) as modulators to phrenic nerve motor recovery; mechanisms that likely occur via adenosine A1 and 5-HT2 receptors, respectively. The present study was designed to specifically determine if concurrent stimulation of 5-HT2 receptors may enhance motor recovery induced by theophylline alone. Adult female rats (250-350 g; n=7 per group) received a left cervical (C2) hemisection that resulted in paralysis of the ipsilateral hemidiaphragm. Twenty-four hours later rats were given systemic theophylline (15 mg/kg, i.v.), resulting in burst recovery in the ipsilateral phrenic nerve. Theophylline-induced recovery was enhanced with the 5-HT2A/2C receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI; 1.0 mg/kg). DOI-evoked augmentation of theophylline-induced recovery was attenuated following subsequent injection of the 5-HT2 receptor antagonist, ketanserin (2.0 mg/kg). In a separate group, rats were pretreated with ketanserin, which did not prevent subsequent theophylline-induced respiratory recovery. However, pretreatment with ketanserin did prevent DOI-induced augmentation of the theophylline-evoked phrenic nerve burst recovery. Lastly, using immunocytochemistry and in situ hybridization, we showed for the first time a positive co-localization of adenosine A1 receptor mRNA and immunoreactivity with phrenic motoneurons of the cervical ventral horns. Taken together, the results of the present study suggest that theophylline may induce motor recovery likely at adenosine A1 receptors located at the level of the spinal cord, and the concurrent stimulation of converging 5-HT2 receptors may augment the response.

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.

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.

Distribution of Serotonin 2A and 2C Receptor mRNA Expression in the Cervical Ventral Horn and Phrenic Motoneurons Following Spinal Cord Hemisection

Cervical spinal cord injury leads to a disruption of bulbospinal innervation from medullary respiratory centers to phrenic motoneurons. Animal models utilizing cervical hemisection result in inhibition of ipsilateral phrenic nerve activity, leading to paralysis of the hemidiaphragm. We have previously demonstrated a role for serotonin (5-HT) as one potential modulator of respiratory recovery following cervical hemisection, a mechanism that likely occurs via 5-HT2A and/or 5-HT2C receptors. The present study was designed to specifically examine if 5-HT2A and/or 5-HT2C receptors are colocalized with phrenic motoneurons in both intact and spinal-hemisected rats. Adult female rats (250-350 g; n = 6 per group) received a left cervical (C2) hemisection and were injected with the fluorescent retrograde neuronal tracer Fluorogold into the left hemidiaphragm. Twenty-four hours later, animals were killed and spinal cords processed for in situ hybridization and immunohistochemistry. Using (35)S-labeled cRNA probes, cervical spinal cords were probed for 5-HT2A and 5-HT2C receptor mRNA expression and double-labeled using an antibody to Fluorogold to detect phrenic motoneurons. Expression of both 5-HT2A and 5-HT2C receptor mRNA was detected in motoneurons of the cervical ventral horn. Despite positive expression of both 5-HT2A and 5-HT2C receptor mRNA-hybridization signal over phrenic motoneurons, only 5-HT2A silver grains achieved a signal-to-noise ratio representative of colocalization. 5-HT2A mRNA levels in identified phrenic motoneurons were not significantly altered following cervical hemisection compared to sham-operated controls. Selective colocalization of 5-HT2A receptor mRNA with phrenic motoneurons may have implications for recently observed 5-HT2A receptor-mediated regulation of respiratory activity and/or recovery in both intact and injury-compromised states.

The spinal cord research foundation: two decades of progress

Over the course of the past 24 years, the Paralyzed Veterans of America's Spinal Cord Research Foundation (SCRF) has provided support for more than 400 research grants in a wide range of areas, from improved wheelchair design to axon pathfinding in Drosophila. The Founders of SCRF, as well as its current trustees, believe that it is imperative to target a broad range of research areas to maximize the quality of life for people, both veterans and nonveterans, with paralysis. This approach has involved the support of basic science and clinical research directed towards repair of the spinal cord, as well as research into improved treatments for complications of spinal cord dysfunction and other projects, including engineering grants and conferences, that may enhance the quality of life for people with paralysis within the immediate future.

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.

Adenosine and neurotrauma: therapeutic perspectives

Cerebral ischemia studies demonstrating that stimulation of adenosine A1 receptors by either endogenously released adenosine or the administration of selective receptor agonists causes significant reductions in the morbidity and mortality associated with focal or global brain ischemias have triggered interest in the potential of purinergic therapies for the treatment of traumatic injuries to the brain and spinal cord. Preliminary findings indicate that activation of A1 adenosine receptors can ameliorate trauma-induced death of central neurons. Other avenues of approach include the administration of agents which elevate local concentrations of adenosine at injury sites by inhibiting its metabolism to inosine by adenosine deaminase, rephosphorylation to adenosine triphosphate by adenosine kinase; or re-uptake into adjacent cells. Amplification of the levels of endogenously released adenosine in such a 'site and event specific' fashion has the advantage of largely restricting the effect of such inhibitors to areas of injury-induced adenosine release. Another approach involving purinergic therapy has been applied to the problem of respiratory paralysis following high spinal cord injuries. In this instance, the adenosine antagonist theophylline has been used to enhance residual synaptic drive to spinal respiratory neurons by blocking adenosine A1 receptors. Theophylline induced, and maintained, hemidiaphragmatic recovery for prolonged periods after C2 spinal cord hemisection in rats and may prove to be beneficial in assisting respiration in spinal cord injury patients.

5-Hydroxytryptophan-induced respiratory recovery after cervical spinal cord hemisection in rats

The present study investigates the role of serotonin in respiratory recovery after spinal cord injury. Experiments were conducted on C(2) spinal cord hemisected, anesthetized, vagotomized, paralyzed, and artificially ventilated rats in which end-tidal CO(2) was monitored and maintained. Before drug administration, the phrenic nerve ipsilateral to hemisection showed no respiratory-related activity due to the disruption of the descending bulbospinal respiratory pathways by spinal cord hemisection. 5-Hydroxytryptophan (5-HTP), a serotonin precursor, was administrated intravenously. 5-HTP induced time- and dose-dependent increases in respiratory recovery in the phrenic nerve ipsilateral to hemisection. Although the 5-HTP-induced recovery was initially accompanied by an increase in activity in the contralateral phrenic nerve, suggesting an increase in descending respiratory drive, the recovery persisted well after activity in the contralateral nerve returned to predrug levels. 5-HTP-induced effects were reversed by a serotonin receptor antagonist, methysergide. Because experiments were conducted on animals subjected to C(2) spinal cord hemisection, the recovery was most likely mediated by the activation of a latent respiratory pathway spared by the spinal cord injury. The results suggest that serotonin is an important neuromodulator in the unmasking of the latent respiratory pathway after spinal cord injury. In addition, the results also suggest that the maintenance of 5-HTP-induced respiratory recovery may not require a continuous enhancement of central respiratory drive.

Effects of serotonin on crossed phrenic nerve activity in cervical spinal cord hemisected rats

The present study investigates the effect of 5-hydroxytryptophan (5-HTP), a serotonin precursor, on crossed phrenic nerve activity (CPNA) in rats subjected to a left C2 spinal cord hemisection. Electrophysiological experiments were conducted on anesthetized, vagotomized, paralyzed, and artificially ventilated rats to assess phrenic nerve activity. The left phrenic nerve lost rhythmic activity due to the disruption of the bulbospinal respiratory pathways following spinal cord hemisection. Activity was induced in the left phrenic nerve (CPNA) by temporary asphyxia. 5-HTP administration increased CPNA during asphyxia in the left phrenic nerve in a dose-dependent fashion. Specifically, in a group of eight animals, application of 5-HTP at 0.5, 1.0, and 2.0 mg/kg significantly increased CPNA by 102.2+/-18.5%, 200.8+/-58.1%, and 615.0+/-356.9% compared with predrug control values, respectively. 5-HTP-induced increases in CPNA were reversed by methysergide (2-6 mg/kg, i.v.), a serotonin receptor antagonist. The results suggest that serotonin is involved in the modulation of crossed phrenic nerve activity following spinal cord injury.