Chronic intermittent hypoxia induces a recoverable impairment in the capacity to elicit inactivity‐induced phrenic motor facilitation (iPMF) following intermittent neural apnea

Abstract

PKCζ dependent increase in phrenic burst amplitude, a form of compensatory plasticity known as inactivity‐induced phrenic motor facilitation (iPMF). We previously suggested that iPMF and related forms of plasticity in other respiratory neurons represent a form of homeostatic plasticity that stabilizes inspiratory motor output and prevents recurrent cessations in breathing. Here, we tested the hypotheses that: 1) chronic intermittent hypoxia (CIH), which is characteristic of sleep apnea, impairs the capacity to elicit iPMF and 2) the capacity to elicit iPMF is restored within days after termination of CIH. Envigo Sprague Dawley rats were exposed to 7 days of CIH (8 hrs/day, 2 min 10.5% O2 separated by 2 min 21% O2) or an equivalent duration of normoxia. Rats were then urethane‐anesthetized and mechanically ventilated; intermittent reductions in respiratory neural activity were induced by briefly hyperventilating below the CO2 apneic threshold for breathing (5, ~1 min neural apnea episodes, separated by 5 min). In rats exposed to 7 days of normoxia, intermittent neural apnea triggered a significant increase in phrenic burst amplitude for up to 60min post‐neural apnea, when compared to time control rats that did not experience reduced respiratory neural activity (49%±6 versus 9±4, %baseline; p<0.05), indicating iPMF. Consistent with our hypotheses, rats exposed to CIH did not express increased phrenic burst amplitude following intermittent neural apnea (0±7, %baseline, p>0.05), indicating CIH impairs iPMF. However, following 7 days recovery from CIH, increased phrenic burst amplitude following intermittent neural apnea was again apparent (56±18, %baseline, p<0.05), suggesting that CIH‐induced impairments in iPMF fully recover when hypoxia is mitigated. To confirm that mechanisms giving rise to iPMF following recovery of CIH are similar to normal rats, subgroups of rats received intrapleural injections of small interfering RNAs directed against PKCζ (100 pg/side) to reduce PKCζ in phrenic motor neurons. As expected, preliminary data indicate that rats receiving non‐targeting siRNA did not express increases in phrenic burst amplitude following intermittent neural apnea immediately following CIH exposure (−10 %baseline), but fully recovered the capacity for intermittent neural apnea to elicit increased phrenic burst amplitude by 7 days post‐CIH exposure (63 %baseline). By contrast, rats treated with intrapleural siPKCζ did not recover the capacity to increase phrenic amplitude following intermittent apnea 7 days following CIH exposure (18±6, %baseline), confirming an essential role for spinal PKCζ in iPMF. Together, these data demonstrate that CIH restrains the expression of iPMF, but that mechanisms of iPMF spontaneously recover over time once hypoxia is resolved. As intermittent reductions in respiratory neural activity and CIH are characteristic of central sleep apnea, we suggest that the occlusion of iPMF by intermittent hypoxia may contribute to pathological sleep apnea.Support or Funding InformationSupported by NIH‐HL105511.