Diaphragm long-term facilitation following acute intermittent hypoxia during wakefulness and sleep

Acute intermittent hypoxia (AIH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). Here, we tested four hypotheses in unanesthetized, spontaneously breathing rats using radiotelemetry for EEG and diaphragm electromyography (Dia EMG) activity: 1) AIH induces LTF in Dia EMG activity; 2) diaphragm LTF (Dia LTF) is more robust during sleep vs. wakefulness; 3) AIH (or repetitive AIH) disrupts natural sleep-wake architecture; and 4) preconditioning with daily AIH (dAIH) for 7 days enhances Dia LTF. Sleep-wake states and Dia EMG were monitored before (60 min), during, and after (60 min) AIH (10, 5-min hypoxic episodes, 5-min normoxic intervals; n = 9), time control (continuous normoxia, n = 8), and AIH following dAIH preconditioning for 7 days (n = 7). Dia EMG activities during quiet wakefulness (QW), rapid eye movement (REM), and non-REM (NREM) sleep were analyzed and normalized to pre-AIH values in the same state. During NREM sleep, diaphragm amplitude (25.1 +/- 4.6%), frequency (16.4 +/- 4.7%), and minute diaphragm activity (amplitude X frequency; 45.2 +/- 6.6%) increased above baseline 0-60 min post-AIH (all P < 0.05). This Dia LTF was less robust during QW and insignificant during REM sleep. dAIH preconditioning had no effect on LTF (P > 0.05). We conclude that 1) AIH induces Dia LTF during NREM sleep and wakefulness; 2) Dia LTF is greater in NREM sleep vs. QW and is abolished during REM sleep; 3) AIH and repetitive AIH disrupt natural sleep patterns; and 4) Dia LTF is unaffected by dAIH. The capacity for plasticity in spinal pump muscles during sleep and wakefulness suggests an important role in the neural control of breathing.