Adenosine A1 Receptor Expression on Phrenic Motor Neurons after Cervical Spinal Injury and Different Intermittent Hypoxia Exposures

Abstract

Exposure to moderate acute intermittent hypoxia (AIH) elicits a serotonin‐dependent, long‐lasting increase in phrenic motor output known as phrenic long‐term facilitation (pLTF). Cervical spinal cord injury (cSCI) disrupts descending projections to the respiratory motor neurons, including descending axons from serotonergic neurons. Serotonergic innervation of the phrenic motor nucleus is partially restored with time post‐cSCI. Nevertheless, persistent pathogenic events post‐SCI (e.g. relative tissue hypoxia and inflammation) may still undermine signaling mechanisms giving rise to pLTF. Understanding these mechanisms is of considerable significance since repetitive exposure to AIH has emerged as a promising therapeutic strategy to restore breathing function after cSCI. Adenosine is another molecule known to both initiate and regulate phrenic motor plasticity. Since both tissue hypoxia and inflammation are associated with increases in extracellular adenosine concentration, the role of adenosine receptor activation in modulating AIH‐induced phrenic motor plasticity after cSCI should be considered. Adenosine 1 and 2A receptors (A1Rs and A2ARs) inhibit and stimulate cAMP production via Gi‐ and Gs‐protein‐coupled signaling cascades, respectively. Since A1Rs and A2ARs compete to determine the outcome of adenosinergic signaling, it is critical to characterize A1Rs expression with chronic cSCI and intermittent hypoxia. With chronic cSCI, A2AR activation undermines therapeutic efficacy of repeated AIH such that combined A2AR antagonism with AIH improves functional recovery. Since prolonged intermittent hypoxia upregulates A2ARs, particularly with cSCI (Smith et al. ibid), we hypothesized that A1Rs are upregulated in a similar manner. Thus, we are evaluating A1R expression in phrenic motor neurons 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 (5‐min 10.5% O2 episodes, 5‐min intervals; 8 hrs/day); and 4) high dose CIH (2‐min 10.5% O2 episodes, 2 min intervals; 8 hrs/day). Rats were then perfused, and cervical spinal cords sectioned (40μm). Four samples were taken per segment from C3 to C5, A1R immunoreactivity is being determined in Cholera toxin B subunit‐labeled phrenic motor neurons using a custom MATLAB algorithm. Preliminary qualitative analyses suggest that A1R expression is reduced by high dose CIH in C2Hx rats. These experiments are the first to explore changes in A1R expression due to intermittent hypoxia and/or cSCI.Support or Funding InformationNIH T32 HD043730 (LLA) OT2OD023854 (SPARC) & 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.