Impact of Intermittent Hypoxia Protocol on Phospho‐p38 and Phospho‐ERK MAP Kinase Expression within Phrenic Motoneurons

Abstract

Acute intermittent hypoxia (AIH) elicits a form of spinal, phrenic motor plasticity known as phrenic long‐term facilitation (pLTF). Preconditioning with daily acute intermittent hypoxia (dAIH) enhances pLTF and increases expression of key plasticity‐associated molecules in the phrenic motor nucleus. On the other hand, preconditioning with chronic intermittent hypoxia (CIH), such as that experienced during sleep apnea, has conflicting effects on pLTF. For example, CIH preconditioning consisting of 5 min hypoxic episodes/5 min intervals (12 hrs/night; 7 nights) enhances AIH‐induced pLTF; in contrast, 8 hrs of CIH consisting of 2 min episodes/2 min intervals abolishes pLTF through a mechanism that requires systemic inflammation and spinal p38 MAP kinase activity. We hypothesized that these differential responses resulted from differences in frequency (6 vs 15/hour) and/or duration (1 vs 7 days) of IH preconditioning. Since there have been no direct comparisons of different IH preconditioning protocols in their effect on pLTF, we recently compared the impact of 3 IH preconditioning protocols presented for 7 consecutive days. Four groups of naïve rats were exposed to either: 1) normoxia (Nx7; 21% O2; 8 hrs/day); 2) dAIH7 (10, 5 min episodes of 10.5% O2, 5 min normoxic intervals); 3) IH7‐5/5 (5 min hypoxic episodes, 5 min intervals; 6 episodes/hr, 8 hrs/day); and 4) IH7‐2/2 (2 min hypoxic episodes, 2 min intervals; 15 episodes/hr, 8 hrs/day). Neurophysiology experiments conducted the day after the final exposure revealed protocol‐dependent effects on AIH‐induced pLTF. With dAIH, pLTF was enhanced; IH7‐5/5 had minimal effect; and IH7‐2/2 abolished pLTF. Here, we present an initial study of neurochemical changes in phrenic motoneurons elicited by these IH preconditioning protocols. We targeted specific kinases that either enable (phospho‐ERK MAP kinase) or inhibit (phospho‐p38 MAP kinase) moderate AIH‐induced pLTF. Rats were injected with cholera toxin B fragment (CTB) and exposed to 7 days of either: 1) Nx7; 2) dAIH7; 3) IH7‐5/5; or 4) IH7‐2/2. Rats were then perfused, and cervical spinal tissue sectioned transversely (40μm), and labeled with antibodies to CTB and either phospho‐p38 or phospho‐ERK. Phospho‐p38 immunofluorescence in phrenic motor neurons was decreased by dAIH (p<0.0001). Unlike with IH‐1 2/2, rats exposed to 7 days of either CIH protocol did not exhibit increased phospho‐p38 expression (IH7‐5/5, p=0.5372; IH7‐2/2, P=0.3924), suggesting a transient effect. Quantification of phospho‐ERK is ongoing. These neurochemical studies will guide future mechanistic studies concerning the mechanisms whereby IH enhances or undermines AIH‐induced pLTF.Support or Funding InformationNIH: K12HD055929 (EGR), R01HL69064 (GSM), T32HD043730 (LLA); DoD: W81XWH‐17‐1‐0315 (EGR); APS (AEH, JVS).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.