Vagal Suppression of Diurnal Variations in Acute Intermittent Hypoxia-Induced Phrenic Long-Term Facilitation

Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF), a sustained increase in phrenic motor output that arises from the balance of competing serotonin- and adenosine-driven mechanisms. Due to shifts in spinal adenosine levels, pLTF exhibits a diurnal cycle: where 15, 1-min hypoxic episodes elicit robust serotonin-dependent pLTF during the diurnal rest phase (~130%) but is markedly attenuated in mid-active phase rats (~30%) due to undermining effects from elevated spinal adenosine. Mechanistic studies of pLTF typically use anesthetized, paralyzed, mechanically ventilated rats with bilateral cervical vagotomy to prevent ventilator entrainment. However, vagotomy increases arterial pressure, which may protect spinal cord perfusion during hypoxic episodes. Here, we tested the hypothesis that intact vagus nerves diminish spinal oxygenation and pLTF expression. Rats with intact vagus nerves had lower arterial pressure and spinal cord oxygen tensions during hypoxic episodes, despite equivalent hypoxemia. Since spinal cord hypoxia promotes adenosine accumulation expected to constrain serotonin-driven pLTF, we hypothesized that intact vagal feedback blunts rest-phase pLTF and abolishes diurnal variations in response to AIH (15, 1-min episodes). With intact vagi, increased phrenic burst amplitude 90 min post-AIH was markedly attenuated in the mid-rest (43±63% baseline), but not mid-active phase (43±25%), suppressing the magnitude of diurnal variation. Cervical spinal delivery of the A2A receptor antagonist MSX-3 restored robust pLTF in both diurnal phases; selective A2A receptor knockdown within phrenic motor neurons enhanced pLTF during mid-rest but not mid-active phase. Thus, intact vagus nerves indirectly shift the spinal serotonin/adenosine balance during hypoxia, suppressing diurnal variations in pLTF magnitude.