Plasticity is an important aspect of the neural control of breathing. One well-studied form of respiratory plasticity is phrenic long-term facilitation (pLTF) induced by acute intermittent but not sustained
hypoxia.
Okadaic acid-sensitive
protein phosphatases (PPs) differentially regulate phrenic nerve activity with intermittent vs. sustained
hypoxia, at least partially accounting for pLTF pattern sensitivity. However,
okadaic acid inhibits multiple
serine/
threonine phosphatases, and the relevant
phosphatase (PP1, PP2A,
PP5) for pLTF pattern sensitivity has not been identified. Here, we demonstrate that sustained
hypoxia (25 min, 9-10.5% O2) elicits phrenic motor facilitation in rats pretreated with bilateral intrapleural
injections of small interfering RNAs (siRNAs; Accell-modified to preferentially transfect neurons, 3.33 μM, 3 days) targeting PP1
mRNA (48 ± 14% change from baseline, n = 6) but not PP2A (14 ± 9% baseline, n = 6) or nontargeting siRNAs (4 ± 10% baseline, n = 7). In time control rats (no
hypoxia) treated with siRNAs ( n = 6), no facilitation was evident (-9 ± 9% baseline). siRNAs had no effect on the hypoxic phrenic response. Immunohistochemistry revealed PP1 and PP2A
protein in identified phrenic motoneurons. Although PP1 and PP2A siRNAs significantly decreased PP1 and PP2A
mRNA in PC12 cell cultures, we were not able to verify "knockdown" in vivo after
siRNA treatment. On the other hand, PP1 and PP2A siRNAs significantly decreased PP1 and PP2A
mRNA in PC12 cell cultures, verifying the intended
siRNA effects. In conclusion, PP1 (not PP2A) is the relevant
okadaic acid-sensitive phosphatase constraining phrenic motor facilitation after sustained
hypoxia and likely contributing to pLTF pattern sensitivity. NEW & NOTEWORTHY This study demonstrates that the relevant
okadaic acid-sensitive Ser/Thr
protein phosphatase (PP) constraining facilitation after sustained
hypoxia is PP1 and not PP2A. It suggests that PP1 may be critical in the pattern sensitivity of
hypoxia-induced phrenic motor plasticity.