Adenosine A1 receptor mRNA expression and the effects of systemic theophylline administration on respiratory function 4 months after C2 hemisection

The phrenic neuromuscular system

The phrenic neuromuscular system consists of the phrenic motor nucleus in the mid-cervical spinal cord, the phrenic nerve, and the diaphragm muscle. This motor system helps sustain breathing throughout life, while also contributing to posture, coughing, swallowing, and speaking. The phrenic nerve contains primarily efferent phrenic axons and afferent axons from diaphragm sensory receptors but is also a conduit for autonomic fibers. On a breath-by-breath basis, rhythmic (inspiratory) depolarization of phrenic motoneurons occurs due to excitatory bulbospinal synaptic pathways. Further, a complex propriospinal network innervates phrenic motoneurons and may serve to coordinate postural, locomotor, and respiratory movements. The phrenic neuromuscular system is impacted in a wide range of neuromuscular diseases and injuries. Contemporary research is focused on understanding how neuromuscular plasticity occurs in the phrenic neuromuscular system and using this information to optimize treatments and rehabilitation strategies to improve breathing and related behaviors.

Sustained A1 Adenosine Receptor Antagonist Drug Release from Nanoparticles Functionalized by a Neural Tracing Protein

Respiratory dysfunction is a major cause of death in people with spinal cord injury (SCI). A remaining unsolved problem in treating SCI is the intolerable side effects of the drugs to patients. In a significant departure from conventional targeted nanotherapeutics to overcome the blood-brain barrier (BBB), this work pursues a drug-delivery approach that uses neural tracing retrograde transport proteins to bypass the BBB and deliver an adenosine A1 receptor antagonist drug, 1,3-dipropyl-8-cyclopentyl xanthine, exclusively to the respiratory motoneurons in the spinal cord and the brainstem. A single intradiaphragmatic injection at one thousandth of the native drug dosage induces prolonged respiratory recovery in a hemisection animal model. To translate the discovery into new treatments for respiratory dysfunction, we carry out this study to characterize the purity and quality of synthesis, stability, and drug-release properties of the neural tracing protein (wheat germ agglutinin chemically conjugated to horseradish peroxidase)-coupled nanoconjugate. We show that the batch-to-batch particle size and drug dosage variations are less than 10%. We evaluate the nanoconjugate size against the spatial constraints imposed by transsynaptic transport from pre to postsynaptic neurons. We determine that the nanoconjugate formulation is capable of sustained drug release lasting for days at physiologic pH, a prerequisite for long-distance transport of the drug from the diaphragm muscle to the brainstem. We model the drug-release profiles using a first-order reaction model and the Noyes-Whitney diffusion model. We confirm via biological electron microscopy that the nanoconjugate particles do not accumulate in the tissues at the injection site. We define the nanoconjugate storage conditions after monitoring the solution dispersion stability under various conditions for 4 months. This study supports further development of neural tracing protein-enabled nanotherapeutics for treating respiratory problems associated with SCI.

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.

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.

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.

Functional recovery in rats with chronic spinal cord injuries after exposure to an enriched environment

BACKGROUND/OBJECTIVE

The objective of this study was to determine the effect of environmental enrichment on the sensorimotor function of rats with chronic spinal cord injuries.

DESIGN

Adult Sprague-Dawley rats received a contusive injury of moderate severity at vertebral level T8 using a weight-drop device. Three months after injury, 1 randomized group (n = 16) of rats was placed in an enriched environment, whereas the control group (n = 16) remained housed in standard laboratory cages (2/cage).

METHODS

Animals were placed in an enriched environment for 4 weeks beginning at 3 months after injury. The enriched environment consisted of a large cage (5-6 rats/cage) with access to items such as tubes, ramps, and running wheel, with items changed daily.

MAIN OUTCOME MEASURES

Functional evaluation consisted of the open field Basso, Beattie and Bresnahan (BBB) locomotor test and the tests that form the combined behavioral score (CBS). The CBS includes motor score, toe spread, placing, withdrawal, righting, inclined plane, hot plate, and swim tests. Behavioral testing was repeated 7 times before and after the period of intervention.

RESULTS

The group placed in the enriched environment scored significantly better on the BBB (ANOVA repeated-measures, P < 0.01) test and CBS (ANOVA repeated-measures, P < 0.01).

CONCLUSIONS

Environmental enrichment results in significant functional improvement in animals with spinal cord injury even with a substantial delay in initiating treatment after injury. The features of an enriched environment that may be responsible for the improvement include social interactions, exercise, and novel items in an interesting environment. These findings suggest a continued plasticity of the chronically injured rat spinal cord and a possible therapeutic intervention for people with spinal cord injury.

Recovery of phrenic activity and ventilation after cervical spinal hemisection in rats

We tested two hypotheses: 1) that the spontaneous enhancement of phrenic motor output below a C2 spinal hemisection (C2HS) is associated with plasticity in ventrolateral spinal inputs to phrenic motoneurons; and 2) that phrenic motor recovery in anesthetized rats after C2HS correlates with increased capacity to generate inspiratory volume during hypercapnia in unanesthetized rats. At 2 and 4 wk post-C2HS, ipsilateral phrenic nerve activity was recorded in anesthetized, paralyzed, vagotomized, and ventilated rats. Electrical stimulation of the ventrolateral funiculus contralateral to C2HS was used to activate crossed spinal synaptic pathway phrenic motoneurons. Inspiratory phrenic burst amplitudes ipsilateral to C2HS were larger in the 4- vs. 2-wk groups ( P < 0.05); however, no differences in spinally evoked compound phrenic action potentials could be detected. In unanesthetized rats, inspiratory volume and frequency were quantified using barometric plethysmography at inspired CO2 fractions between 0.0 and 0.07 (inspired O2 fraction 0.21, balance N2) before and 2, 3, and 5 wk post-C2HS. Inspiratory volume was diminished, and frequency enhanced, at 0.0 inspired CO2 fraction ( P < 0.05) 2-wk post-C2HS; further changes were not observed in the 3- and 5-wk groups. Inspiratory frequency during hypercapnia was unaffected by C2HS. Hypercapnic inspiratory volumes were similarly attenuated at all time points post-C2HS ( P < 0.05), thereby decreasing hypercapnic minute ventilation ( P < 0.05). Thus increases in ipsilateral phrenic activity during 4 wk post-C2HS have little impact on the capacity to generate inspiratory volume in unanesthetized rats. Enhanced crossed phrenic activity post-C2HS may reflect plasticity associated with spinal axons not activated by our ventrolateral spinal stimulation.

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.