Vagal Control of Breathing Pattern after Midcervical Contusion in Rats

The present study was designed to establish a midcervical contusion model that can simulate long-term respiratory deficits, and investigate the breathing pattern during vagal-mediated respiratory reflexes following midcervical contusion. Moderate and severe (impactor height: 6.25 or 12.5 mm) contusion was induced at midline C3-4 spinal cord in adult Sprague-Dawley rats. The ventilatory behaviors of unanesthetized were evaluated by whole body plethysmography at 1 day and 1, 2, 4, and 8 weeks post-injury. The tidal volume was decreased and frequency was increased in contused animals compared with uninjured animals at the acute injury state. At 8 weeks post-injury, respiratory frequency was similar between groups; however, contused animals had lower tidal volume. The pulmonary chemoreflex induced by intrajugular capsaicin (1.5 μg/kg) injection and the Hering-Breuer inflation reflex induced by increasing positive end-expired pressure (9 cm H₂O) were evoked in anesthetized animals at 3 days, or 2 or 8 weeks post-surgery. The result showed that capsaicin induced a significant prolongation of the expiratory duration in uninjured animals; however, this response was greatly attenuated in contused animals at 3 days post-injury. Increased positive end-expired pressure also caused an increase in the expiratory duration in uninjured and moderately contused animals; however, severely contused animals exhibited an attenuated response. At 2 and 8 weeks post-injury, both the pulmonary chemoreflex and the Hering-Breuer inflation reflex were similar between uninjured and contused animals. These data suggested that midcervical contusion can cause a long-term respiratory impairment and a transiently attenuation of vagal-mediated respiratory reflexes.