Pattern of Acute Intermittent Hypoxia Protocol Impacts Phrenic Long-term Facilitation
Authors: Marciante Alexandria , Seven Yasin , Perim Raphael , Mitchell Gordon 
Published: May 2023

An important model of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a prolonged increase in phrenic nerve activity following intermittent exposure to low oxygen (acute intermittent hypoxia, AIH). pLTF arises from distinct intracellular signaling cascades initiated by serotonin 2 and adenosine 2A (A2A) receptor activation, respectively, that interact via powerful crosstalk inhibition. Studies of AIH-induced motor plasticity typically use moderate AIH protocols consisting of 3, 5 min hypoxic episodes in rodent models, but 15, 1 min hypoxic episodes in humans. Since certain factors ( e.g. severity or duration of hypoxia) influence pLTF expression, it is essential to understand the influence of AIH protocol details on motor plasticity. Here we tested the hypothesis that serotonin-dominant plasticity is enhanced when the duration of hypoxic episodes in the AIH protocol is shorter due to less hypoxia-evoked adenosine release, eliminating adenosine constraint and revealing enhanced plasticity. Anesthetized, vagotomized, paralyzed and ventilated male Sprague-Dawley rats were exposed to different AIH protocols with the same cumulative duration (15 min) and severity (arterial Po2 = 40-55mmHg) of hypoxic episodes: 1) 3, 5 min episodes with 5 min intervals (3x5); and 2) 15, 1 min hypoxic episodes with 1 min intervals (15x1). Consistent with prior reports, 3x5 AIH elicits ~60% pLTF (56±5%; n=7). In contrast, 15x1 AIH elicits significantly greater pLTF (139±14%; n=7; p<0.001). To determine if this effect is due to differences in hypoxia evoked-adenosine release in the ventral C4 spinal cord, tissue oxygen pressure and spinal adenosine concentration were measured with an oxygen optode, and adenosine probe, respectively. Although tissue oxygen pressure between protocols was not different, hypoxia-evoked adenosine release was substantially greater with the 3x5 vs. 15x1 AIH protocols (p<0.001; both n=3). To verify pLTF magnitude results from differential spinal A2A receptor activation, rats were pretreated with a selective A2A receptor antagonist, MSX-3 (10uM; 12 uL) delivered intrathecally at C4; with the 3x5 AIH protocol, pLTF was enhanced as expected (141±15%; p<0.001; n=7). However, MSX-3 had no effect on pLTF elicited by the 15x1 AIH protocol (122±11%; n=7; p=0.931). Thus, the mechanism driving pLTF is serotonin-dominant and adenosine-constrained with 3x5 AIH, but adenosine- unconstrained with the 15x1 AIH protocol. We conclude that, by shortening hypoxic episodes from 5 to 1 min (with the same cumulative hypoxic duration), reduced hypoxia-evoked adenosine release reveals the full extent of serotonin-driven plasticity. AIH is emerging as a promising therapeutic modality to restore respiratory and non-respiratory movements in people with spinal injury and ALS. Thus, knowledge that the AIH protocol can be optimized by reducing the length of individual hypoxic episodes has important experimental, biological and translational implications. Supported by: NIH R01HL148030, R01HL149800 & T32HL134621 (ABM), and UF Brain Spinal Cord Research Trust Fund (BSCIRTF) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.