Neurochemical Plasticity of Phrenic Motor Neuron Adenosine 2A Receptors: Effects of Cervical Spinal Injury and Intermittent Hypoxia

Abstract

Cervical spinal adenosine 2A receptor activation is necessary for certain forms of spinal respiratory motor plasticity, such as phrenic long‐term facilitation elicited by severe acute intermittent hypoxia (AIH; Nichols et al., 2012) or moderate AIH with acute cervical spinal injury (cSCI; Navarette‐Opazo et al., 2016). Whereas cervical spinal A2A receptor activation is sufficient to elicit phrenic motor facilitation (pMF; Golder et al., 2008), it undermines other, serotonin‐dependent forms of pMF via cross‐talk inhibition (Hoffman et al., 2010; Navarette‐Opazo et al., 2017). Thus, A2A receptors exert complex effects on phrenic motor plasticity. However, it is not known if conditions such as cSCI and/or repetitive exposure to intermittent hypoxia elicit neurochemical plasticity in A2A receptor expression (and associated phrenic motor plasticity). Thus, we characterized A2A receptor expression in phrenic motor neurons after chronic cSCI and prolonged exposure to different protocols of intermittent hypoxia. A2A receptor expression was determined in male Sprague Dawley rats with and without C2 spinal hemisection (C2Hx; 12 wks post‐injury) that were exposed to 28 days of: 1) normoxia; 2) daily AIH (10, 5‐min 10.5% O2 episodes with 5‐min intervals per day; 1.5 hrs/day); 3) moderate CIH (CIH 5/5: 5‐min 10.5% O2 episodes, 5‐min intervals; 8 hrs/day); and 4) high dose CIH (CIH 2/2: 2‐min 10.5% O2 episodes, 2 min intervals; 8 hrs/day). Rats were then perfused, and cervical spinal cords sectioned (40μm). C3 to C5 sections were processed for A2A receptor and Cholera toxin B subunit (CtB, injected 14 days before C2Hx). Using a custom MATLAB algorithm, optical density in CtB‐positive phrenic motor neurons was quantified. Statistical analysis was performed using MANOVA (SAS JMP). C2Hx decreased A2A receptor expression in phrenic motor neurons (p<0.05). Intermittent hypoxia exerted protocol‐specific effects on A2A receptor expression (CIH 2/2 > CIH 5/5 > daily AIH; p=0.014). Although intermittent hypoxia appeared to increase A2A receptor expression more after C2Hx, this effect was only marginally significant (injury/treatment interaction; p=0.061). The dual effects of injury and intermittent hypoxia on A2A receptor expression suggest that adenosine receptor plasticity may be important in the development of new therapeutic strategies to improve breathing after spinal cord injury.Support or Funding InformationSupported by: NIH T32 HD043730 (LLA) OT2OD023854 (SPARC), NIH K12 HD055929 (EGR), McKnight Brain Institute.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.