Time of Day Effects on Phrenic Motor Plasticity in Female Rats during Estrus

Moderate acute intermittent hypoxia (mAIH) elicits a form of spinal, respiratory motor plasticity known as phrenic long-term facilitation (pLTF). mAIH activates competing intracellular signaling cascades, initiated by serotonin 2 and adenosine 2A (A2A) receptors, respectively. Using our historically “standard” mAIH protocol (3, 5 min moderate hypoxic episodes), pLTF magnitude is similar in the rest and active phase in male rats. However, the mechanism driving plasticity shifts from serotonin- (mid-rest phase) to adenosine- (mid-active phase) dominant due to normal diurnal changes in basal spinal adenosine levels. Basal adenosine also increases with age in males, but it is unknown if similar age and/or diurnal shifts occur in females. We hypothesized that young (3-4 months) and aged (16-18 months) female Sprague Dawley rats have elevated basal spinal adenosine levels during estrus (when estrogen is lowest), thereby undermining rest phase mAIH-induced pLTF (3,5-min episodes; arterial PO 2 =45-55mmHg) and enhancing active phase pLTF. In young females, spinal A2A receptor inhibition enhanced rest phase pLTF (108±23%; n=3) vs vehicle controls (20±10%; n=3; p<0.001). In aged females, rest phase pLTF was elevated (53±7%; n=3) vs young females, consistent with prior reports; further, spinal A2A receptor inhibition increased pLTF to higher levels vs young females (173±24%; n=3; p<0.001). Rest phase basal adenosine levels in ventral C3-C5 homogenates were elevated 2-3-fold in young females during estrus vs aged females, which exhibit persistent estrus (p=0.003). In the active phase, pLTF was significantly elevated from rest phase levels in both young (188±12%; n=4) and aged (155±22%; n=3) females. However, in contrast to rest phase, active phase pLTF was nearly abolished by spinal A2A receptor inhibition in both young and aged females. Understanding age, time-of-day effects and phase of the estrous cycle on AIH-induced respiratory motor plasticity in females have important biological implications that must be considered as we translate and develop new therapies for severe neuromuscular disorders that compromise breathing and other movements. 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.