Adenosine is a Key Regulator of Intermittent Hypoxia-Induced Phrenic Motor Plasticity

Acute intermittent hypoxia (AIH) elicits phrenic long-term facilitation (pLTF), a well-studied model of respiratory motor plasticity expressed as a persistent increase in phrenic nerve activity lasting hours post-AIH. With moderate AIH (mAIH), hypoxia increases brainstem raphe neuron activity, releasing serotonin in the phrenic motor nucleus and driving plasticity. Slower development of spinal tissue hypoxia within hypoxic episodes evokes spinal adenosine formation that constrains serotonin-induced plasticity. Beyond hypoxia-evoked adenosine formation, multiple factors regulate basal spinal adenosine levels such as time-of-day, mild systemic inflammation and aging. Ultimately, the extent of mAIH-induced pLTF depends on the spinal serotonin versus adenosine balance. Years of study of these fundamental mechanisms inspired clinical trials of mAIH to elicit plasticity and improve breathing and non-respiratory movements in people with spinal cord injury, ALS, MS and stroke. However, in all published human trials to date, pronounced functional improvements are observed in only ~60% of subjects, with little response in 40%. We hypothesize that an unfavorable spinal serotonin/adenosine balance accounts for these “low responders”. To begin investigation of this hypothesis, we performed a metanalysis of serotonin-driven pLTF and spinal adenosine data collected from multiple studies on male and female young (3-6 month), middle-aged (12-15 month) and old (22-24 month) Sprague-Dawley rats, and variable protocols including LPS-induced systemic inflammation, mid-rest vs mid-active mAIH delivery, duration of hypoxic episodes within AIH protocol, and an APOE4 allele. A highly significant, inverse sigmoidal relationship was revealed between basal spinal adenosine levels and pLTF magnitude in AIH protocols consisting of either shorter (15x1 min; p=0.020) or longer (3x5 min; p=0.009) moderate hypoxic episodes. However, the sigmoidal relationship is steeper in the 15x1 ( vs 3x5) min protocol, likely due to minimal hypoxia-evoked adenosine accumulation with shorter hypoxic episodes. In all conditions, spinal adenosine 2A receptor inhibition prior to mAIH significantly increased pLTF, regardless of basal spinal adenosine level. It is critical to understand factors affecting basal and hypoxia-evoked adenosine accumulation in regulating mAIH-induced pLTF. Greater understanding of these factors may help identify the individuals least/most likely to respond to therapeutic AIH and suggest at least one solution to alleviate the adenosine constraint to plasticity and functional recovery. NIH K99HL175029 (ABM), Parker B. Francis Foundation (ABM), NIH R01HL149800 (GSM) and NIH R01HL148030 (GSM). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.