Daily acute intermittent hypoxia enhances phrenic motor output and stimulus-evoked phrenic responses in rats

Plasticity is a hallmark of the respiratory neural control system. Phrenic long-term facilitation (pLTF) is one form of respiratory plasticity characterized by persistent increases in phrenic nerve activity following acute intermittent hypoxia (AIH). Although there is evidence that key steps in the cellular pathway giving rise to pLTF are localized within phrenic motor neurons (PMNs), the impact of AIH on the strength of breathing-related synaptic inputs to PMNs remains unclear. Furthermore, the functional impact of AIH is enhanced by repeated/daily exposure to AIH (dAIH). Here, we explored the effects of AIH versus 2 wk of dAIH preconditioning on spontaneous and evoked phrenic responses in anesthetized, paralyzed, and mechanically ventilated rats. Evoked phrenic potentials were elicited by respiratory cycle-triggered lateral funiculus stimulation at the C2 spinal level delivered before and 60 min post-AIH (or the equivalent in time controls). Charge-balanced biphasic pulses (100 μs/phase) of progressively increasing intensity (100–700 μA) were delivered during the inspiratory and expiratory phases of the respiratory cycle. Although robust pLTF (∼60% from baseline) was observed after a single exposure to moderate AIH (3 × 5 min; 5-min intervals), there was no effect on evoked phrenic responses, contrary to our initial hypothesis. However, in rats preconditioned with dAIH, baseline phrenic nerve activity and evoked responses were increased, suggesting that repeated exposure to AIH enhances functional synaptic strength when assessed using this technique. The impact of daily AIH preconditioning on synaptic inputs to PMNs raises interesting questions that require further exploration.