High frequency epidural stimulation across the respiratory cycle evokes phrenic short-term potentiation after incomplete cervical spinal cord injury

C2 spinal hemilesion (C2Hx) paralyzes the ipsilateral diaphragm, but recovery is possible through activation of "crossed spinal" synaptic inputs to ipsilateral phrenic motoneurons. We tested the hypothesis that high frequency epidural stimulation (HF-ES) would potentiate ipsilateral phrenic output after subacute and chronic C2Hx. HF-ES (300 Hz) was applied to the ventrolateral C4 or T2 spinal cord ipsilateral to C2Hx in anesthetized and mechanically ventilated adult rats. Stimulus duration was 60-sec and currents ranged from 100-1000 µA. Bilateral phrenic nerve activity and ipsilateral hypoglossal nerve activity were recorded before and after HF-ES. Higher T2 stimulus currents potentiated ipsilateral phasic inspiratory activity at both 2- and 12-wks post-C2Hx, while higher stimulus currents delivered at C4 potentiated ipsilateral phasic phrenic activity only at 12-wks (P=0.028). Meanwhile, tonic output in the ipsilateral phrenic nerve reached 500% of baseline values at the high currents with no difference between 2- and 12-wks. HF-ES did not trigger inspiratory burst frequency changes. Similar responses occurred following T2 HF-ES. Increases in contralateral phrenic and XII nerve output were induced by C4 and T2 HF-ES at higher currents, but the relative magnitude of these changes was small compared to the ipsilateral phrenic response. We conclude that following incomplete cervical SCI, HF-ES of the ventrolateral mid-cervical or thoracic spinal cord can potentiate efferent phrenic motor output with little impact on inspiratory burst frequency. However, the substantial increases in tonic output indicate that the uninterrupted 60-sec stimulation paradigm used here is unlikely to be useful for respiratory muscle activation after spinal injury.

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