Phrenic long-term facilitation (pLTF) is a form of respiratory plasticity elicited by acute intermittent hypoxia (AIH). AIH activates competing intracellular signaling cascades, initiated by phrenic motor neuron serotonin 2 and adenosine 2A (A2A) receptors, respectively. With moderate AIH (mAIH), the serotonin-dependent Q pathway to phrenic motor facilitation dominates, with an adenosine constraint. Consequently, spinal A2A receptor inhibition enhances mAIH-induced pLTF. In middle-aged male, and geriatric acyclic female rats, pLTF is attenuated through unknown mechanisms. We hypothesize that basal spinal adenosine levels are elevated in aged rats, thereby undermining mAIH-induced pLTF. Thus, we predict that spinal A2A receptor inhibition in aged male rats will restore pLTF to levels equal to young male rats with A2A receptor inhibition. A selective A2A receptor antagonist (MSX-3; 10 uM, 12 uL) or vehicle (0.9% sterile saline) were delivered intrathecally (C4) in anesthetized, vagotomized, paralyzed and ventilated young (3-6 months) and aged (20-22 months) male Sprague-Dawley rats prior to mAIH (3, 5 min hypoxic episodes; arterial Po2 = 45-55 mmHg). In young males, mAIH elicits ~ 60% pLTF at 60 min post-AIH. Spinal A2A receptor inhibition significantly enhanced mAIH-induced pLTF in young males (119 + 5%; n = 6) vs vehicle controls (55 + 9%; n = 6; p lt; 0.001) and time controls (12 + 4%; n = 6; p lt; 0.001). In aged males, A2A receptor inhibition enhanced mAIH-induced pLTF to a greater extent (186 + 19%; n = 7) vs vehicle controls (25 + 11%; n = 7; p lt; 0.001) and time controls (3 + 5%; n = 4; p lt; 0.001). Measured basal adenosine levels in ventral C3-C5 homogenates were significantly elevated in aged vs young males (p = 0.008). Preliminary data suggest aged female rats (16-18 months) also have enhanced mAIH-induced pLTF with A2A receptor inhibition (173+ 24%; n = 3) vs vehicle controls (53 + 7%; n = 3) and time controls (5 + 17%; n = 2) in persistent estrus. These findings advance our understanding of mechanisms whereby age undermines mAIH-induced phrenic motor plasticity. Understanding age and sex effects on AIH-induced respiratory motor plasticity is crucial as we harness therapeutic AIH to treat severe neuromuscular disorders that compromise breathing.
