Daily ketoprofen restores AIH‐induced phrenic long‐term facilitation after prolonged chronic intermittent hypoxia in spinal intact and chronically injured rats

Abstract

Acute intermittent hypoxia (AIH) elicits a form of spinal, respiratory motor plasticity known as phrenic long‐term facilitation (pLTF). In recent years, this form of plasticity has been leveraged for therapeutic benefit to restore respiratory (and non‐respiratory) motor function after spinal cord injury (SCI). While therapeutic AIH shows tremendous promise as a non‐invasive and easy‐to administer rehabilitation strategy, its efficacy may be undermined by factors such as inflammation (Huxtable et al., 2011). For example, a single night of chronic intermittent hypoxia (CIH) simulating that experienced by an individual with moderate sleep apnea (2 min hypoxic episodes; 2 min intervals; 8 hrs) abolishes AIH‐induced pLTF through a mechanism that requires systemic inflammation and spinal p38 MAP kinase activity (Huxtable et al., 2015). Similarly, prolonged CIH (7‐28 days) also abolishes pLTF (Gonzalez‐Rothi et al., 2021; El Chami and colleagues, unpublished), though the underlying mechanisms have not yet been defined. Since CIH induces time‐dependent cascades of distinct spinal inflammatory gene expression (Smith et al., 2013), we hypothesize that prolonged CIH (versus a single night) blunts pLTF through an inflammation‐dependent mechanism, although the specific pathways may differ with cumulative/repeated exposures. Further, since respiratory insufficiency is a leading cause of morbidity and mortality following cervical SCI and sleep disordered breathing/sleep apnea are highly prevalent (nearly 80% in cervical SCI), it is important to understand how untreated sleep apnea may interact with and/or undermine the efficacy of therapeutic interventions like AIH in chronic SCI. Thus, we are exploring the impact of anti‐inflammatory administration on AIH‐induced pLTF in both intact rats and rats with chronic (12 wks) C3/C4 mid‐cervical spinal cord contusion, preconditioned with 7 days of moderate CIH. In intact rats, daily administration of the non‐steroidal anti‐inflammatory drug, ketoprofen rescued expression of pLTF following prolonged CIH (70.4% increase from baseline, p=0.0039 vs. vehicle). Interestingly, a single dose of ketoprofen delivered just prior to pLTF assessments did not restore expression of plasticity (19.5% increase, p=0.8842 vs. vehicle), suggesting a shift in the underlying mechanisms by which CIH impacts phrenic motor plasticity that occurs with cumulative exposure over time. Similarly, in rats with chronic cSCI, daily ketoprofen rescued expression of pLTF following prolonged CIH (62.9% increase from baseline, p=0.0348 vs. vehicle). Upcoming experiments will explore the role of p38 MAP kinase specifically after prolonged CIH. Improved understanding of factors that undermine plasticity, (e.g., CIH, inflammation), will enable us to identify strategies to mitigate deleterious effects and optimize functional benefits, which is essential for effectively translating promising interventions like therapeutic AIH to clinical practice.