Using Caffeine to Enhance the Therapeutic Effects of Respiratory Training After Cervical Spinal Cord Injury.

Cervical spinal cord injury (SCI) often leads to damage of the phrenic motor circuit that innervates the diaphragm, resulting in life-threatening respiratory deficits. While there is some limited spontaneous plasticity after a cervical SCI, severe respiratory deficits persist. Previous research has shown that non-invasive respiratory training with intermittent hypoxia (IH) can enhance respiratory plasticity after SCI. There are two prominent plasticity promoting pathways associated with IH training, dependent on serotonin or adenosine. Prior work suggests there is greater activation of the serotonergic plasticity-promoting pathway after inhibiting the adenosine pathway. Therefore, this work aims to use a clinically relevant adenosine antagonist - caffeine - in combination with IH respiratory training to further enhance the respiratory plasticity and diaphragm recovery of adult female rats after a mid-cervical (C3/4) contusion injury. We hypothesized that 4 weeks of daily IH training primed with 8mg of caffeine, delivered 30 minutes before the training, will enhance the serotonergic plasticity-promoting pathway to stimulate both anatomical and functional phrenic plasticity and improve respiration post-SCI. To label phrenic circuitry, 72 hours before terminal experiments, animals were traced with a retrograde, transsynaptic tracer, pseudorabies virus (PRV), applied to the ipsilateral hemidiaphragm to label phrenic moto- and interneurons. Anatomical plasticity was assessed using immunohistochemistry to quantify interneuronal connectivity and density of descending serotonergic input to the phrenic motor network. Specifically, this study also investigated the connectivity of a cholinergic interneuron subpopulation within phrenic circuitry after IH training and caffeine delivery. Functional plasticity and respiratory recovery after IH training primed with caffeine were assessed with weekly whole-body plethysmography and terminal diaphragm electromyography. The results of this study using caffeine as a primer for respiratory training can be easily translated to improve training outcomes and promote greater plasticity and recovery after SCI.