Activation of phrenic afferents increases phrenic motor output and induces short-term plasticity. Here we tested the hypothesis that activation of phrenic afferents via acute diaphragm stimulation induces neuromuscular plasticity within the phrenic motor system. Bilateral EMG activity was recorded from the mid-costal diaphragm of anesthetized and spontaneously breathing rats. Inspiratory-triggered stimulation was delivered to the left diaphragm for 30 minutes at twitch threshold (n=7; 0.6 ± 0 mA) or 1.5X twitch threshold (n=7; 1.0 ± 0.12 mA), and output was recorded for 60 minutes post-stimulation. Arterial blood gas samples revealed that PaCO2 was no different than baseline during or after stimulation (P=0.2204) or across experimental groups (P=0.3534). Diaphragm amplitude ipsilateral and contralateral to stimulation was increased 60 minutes following threshold (P=0.0021 and P=0.0096) and 1.5X threshold (P=0.0003 and P=0.0448) stimulation compared to time controls (n=7). Separate groups of rats received 1.5X threshold stimulation and either a C3-C6 laminectomy (n=6; 0.8 ± 0.06mA) or a left C3-C6 dorsal rhizotomy (n=6; 0.7 ± 0.10mA) to prevent afferent input from reaching the spinal cord. Laminectomy controls continued to express neuromuscular plasticity ipsilateral (P=0.0017) and contralateral (P=0.0076) to stimulation compared to time controls. By contrast, following unilateral dorsal rhizotomy, ipsilateral and contralateral diaphragm amplitude 60 minutes post-stimulation was lower than laminectomy controls (P=0.0107 and P=0.0115) and no different than time controls (P=0.5692 and P=0.4784), suggesting neuromuscular plasticity was not induced. Our results indicate that acute diaphragm stimulation induces a novel form of neuromuscular plasticity which requires activation of phrenic afferents. We hypothesize that phrenic afferent induced neuromuscular plasticity may be valuable as a therapeutic strategy following spinal cord injury.
