Locomotor Training Enhances Respiratory Plasticity in a Pre‐clinical Model of Cervical Spinal Cord Injury

Abstract

Spinal cord injury (SCI) is a devastating and irreversible injury that impacts thousands of people each year. Most injuries occur at cervical levels, compromising the phrenic motor network, resulting in life‐threatening respiratory deficits. While there is mounting clinical and pre‐clinical evidence for spontaneous plasticity, it is limited, and life‐threatening deficits persist. One strategy for therapeutically enhancing plasticity is the use of activity‐based therapies, such as respiratory and locomotor training. Clinical studies using treadmill training have shown improved respiratory function for individuals with SCI. Therefore, the present work reverse translates these promising clinical findings back to animal models of mid‐cervical SCI to assess the effects of treadmill training on anatomical and functional respiratory plasticity after injury. Adult female Sprague‐Dawley rats (~200g) received a lateralized C3/4 contusive injury. One week post injury, the animals were blindly divided into two groups: untrained or treadmill trained (30 min, once a day, 5 days a week) for 4 weeks. Breathing behavior was measured each week using whole‐body plethysmography. After training completion, all animals were anatomically traced with retrograde and transsynaptic pseudorabies virus (PRV) to the left hemidiaphragm to label the phrenic motor network, ipsilateral to injury. Three days later, animals were anesthetized and diaphragm electromyography (dEMG) was performed to observe ipsilateral diaphragm deficits and respiratory recovery, and animals were perfused for histology. Immunohistological assessment was used to quantify interneuronal connectivity and density of descending serotonergic input to the phrenic motor network. Preliminary data demonstrated increased interneuronal connectivity with injured phrenic motor network, and increased serotonergic input to spinal networks caudal to injury. Electrophysiological assessment of diaphragm function revealed increased diaphragm activity ipsilateral to injury, after 4 weeks of training. Ongoing analyses will further elucidate the therapeutic effects of locomotor training for improving breathing post‐SCI.