Actions of Systemic Theophylline on Hemidiaphragmatic Recovery in Rats Following Cervical Spinal Cord Hemisection

This study assesses the effects of theophylline on enhancing phrenic nerve discharge and functional hemidiaphragmatic recovery after C2 spinal cord hemisection in adult female rats. There were three separate groups of spinal hemisected rats and one nonhemisected group studied. Twenty-four hours following C2 spinal hemisection, ipsilateral phrenic nerve activity was recorded under standardized, normoxic and then hypoxic conditions. After 30 min, theophylline was administered and the recordings were repeated in group 1 animals. In group 2, activity in both phrenic nerves was recorded simultaneously before and after drug administration. In a third group of rats, both ipsilateral phrenic nerve and hemidiaphragmatic activities were monitored before and after the drug. In control nonhemisected animals under standardized recording conditions, the effects of theophylline were quantitatively assessed by determining the mean area under integrated phrenic nerve discharge waveforms before and after drug administration. Generally, theophylline induced biphasic effects; i.e., at a low dose (15 mg/kg) it evoked excitation, while at a high dose (30 mg/kg) depression of respiratory activity predominated. In group 2 animals, respiratory activity was induced in the nerve ipsilateral to the hemisection and enhanced in the contralateral phrenic nerve for up to 3 h after a single standard dose of theophylline (15 mg/kg). Prior to drug administration, there was an absence of respiratory-related activity in both the phrenic nerve and hemidiaphragm ipsilateral to C2 spinal cord hemisection. A standard dose of theophylline, however, induced recovery of activity in both the phrenic nerve and the left hemidiaphragm ipsilateral to the hemisection in group 3 animals. In control (nonhemisected) animals, theophylline enhanced phrenic nerve activity, but decreased the duration of respiratory bursts. These results show for the first time that theophylline can activate latent respiratory motor pathways and thus restore the respiratory drive to phrenic motoneurons lost by spinal cord injury. Respiratory activity is not only reestablished in the phrenic nerve made quiescent by hemisection, but it is also enhanced in the contralateral phrenic nerve. The drug also restores function to the hemidiaphragm paralyzed by the spinal cord hemisection. The findings may have clinical relevance to human cases of cervical spinal cord injury in which respiratory function is compromised.