Long Term Closed‐Loop Cervical Epidural Stimulation Elicits Plasticity in Diaphragm Motor Output

Abstract

More than half of traumatic spinal cord injuries (SCI) occur in the cervical region, leading to respiratory compromise and even failure. The leading cause of mortality after SCI is respiratory failure; thus there is a critical need for new strategies to preserve and/or restore breathing ability after cervical SCI. Most spinal injuries are incomplete; a promising strategy to restore function is to harness the intrinsic capacity for spinal respiratory plasticity, strengthening spared pathways to respiratory motor neurons. Recent reports demonstrate the potential of epidural stimulation to improve functional outcomes after SCI (Wagner et al., 2018). However, in patients with chronic SCI, the persistence of these functional improvements is not well characterized. We have developed a closed‐loop epidural stimulation protocol in freely behaving rats that utilizes endogenous respiratory output from diaphragm electromyography (EMG) to trigger electrical stimulation through bilateral epidural electrodes at C4. Long‐term (20 h/d, 4 d), closed‐loop epidural stimulation elicits a dramatic potentiation of evoked responses in ipsilesional and contralesional diaphragm (Dale et al., 2019). Additionally, ipsilesional peak‐to‐peak amplitude increases after stimulation with additional peaks appearing in longer time domains, which suggests a potential for interneuronal recruitment. Correlative RTqPCR analysis has suggested upregulation of key neurotrophic factors (e.g., BDNF, VEGF, NT3) in cervical spinal cord after closed‐loop stimulation. Thus, we hypothesize that facilitation of evoked motor output from diaphragm is dependent on neurotrophic factor receptor activation within the phrenic motor network. Five rat groups were studied: 1) C2 hemisection (C2HS) + stim (n = 3); 2) C2HS + stim + intrapleural siTrkB (n = 1); 3) C2HS + no stim (n = 6); 4) no injury + stim (n = 2); and 5) sham controls (n = 2). Increased current during motor evoked potential (MEP) testing corresponded with an expected increase in area under the curve (AUC) of the stimulus triggered average in all rats (range: 50–750 mA or max tolerated; ~0.5 Hz inspiratory‐triggered), but 4 days of closed‐loop stimulation robustly facilitates this response in stimulated vs. non‐stimulated controls. Additional preliminary evidence suggests that siRNA targeted to TrkB (the high affinity receptor to BDNF) in phrenic motor neurons abolishes closed‐loop stimulation‐induced facilitation of diaphragm motor output indicating this form of plasticity may rely on canonical neurotrophic factors involved in respiratory neuroplasticity. In addition, longitudinal characterization of the foreign body response to implanted electrodes indicates that further analysis of complex impedance spectra is warranted. These preliminary results may provide detailed mechanistic insights concerning the potential of closed‐loop epidural stimulation to elicit respiratory neuroplasticity, thereby addressing key issues in a critical homeostatic control system necessary for life itself.Support or Funding InformationCraig H. Nielsen Foundation