Intermittent Hypoxia-Induced Spinal Inflammation Impairs Respiratory Motor Plasticity by a Spinal p38 MAP Kinase-Dependent Mechanism

Abstract

Inflammation is characteristic of most clinical disorders that challenge the neural control of breathing. Since inflammation modulates neuroplasticity, we studied the impact of inflammation caused by prolonged intermittent hypoxia on an important form of respiratory plasticity, acute intermittent hypoxia (three, 5 min hypoxic episodes, 5 min normoxic intervals) induced phrenic long-term facilitation (pLTF). Because chronic intermittent hypoxia elicits neuroinflammation and pLTF is undermined by lipopolysaccharide-induced systemic inflammation, we hypothesized that one night of intermittent hypoxia (IH-1) elicits spinal inflammation, thereby impairing pLTF by a p38 MAP kinase-dependent mechanism. pLTF and spinal inflammation were assessed in anesthetized rats pretreated with IH-1 (2 min hypoxia, 2 min normoxia; 8 h) or sham normoxia and allowed 16 h for recovery. IH-1 (1) transiently increased IL-6 (1.5 Ϯ 0.2-fold; p ϭ 0.02) and inducible nitric oxide synthase (iNOS) (2.4 Ϯ 0.4-fold; p ϭ 0.01) mRNA in cervical spinal homogenates, (2) elicited a sustained increase in IL-1␤ mRNA (2.4 Ϯ 0.2-fold; p Ͻ 0.001) in isolated cervical spinal microglia, and (3) abolished pLTF (Ϫ1 Ϯ 5% vs 56 Ϯ 10% in controls; p Ͻ 0.001). pLTF was restored after IH-1 by systemic NSAID administration (ketoprofen; 55 Ϯ 9%; p Ͻ 0.001) or spinal p38 MAP kinase inhibition (58 Ϯ 2%; p Ͻ 0.001). IH-1 increased phosphorylated (activated) p38 MAP kinase immunofluorescence in identified phrenic motoneurons and adjacent microglia. In conclusion, IH-1 elicits spinal inflammation and impairs pLTF by a spinal p38 MAP kinase-dependent mechanism. By targeting inflammation, we may develop strategies to manipulate respiratory motor plasticity for therapeutic advantage when the respiratory control system is compromised (e.g., sleep apnea, apnea of prematurity, spinal injury, or motor neuron disease).

The impact of inflammation on respiratory plasticity

“…Eight hours of intermittent hypoxia (2 min episodes, 10.5% O 2 , followed by 16 hours of normoxia (IH-1) eliminated pLTF but was restored after systemic NSAID administration (Huxtable et al, 2015). Furthermore, IH-1 was shown to upregulate gene expression of IL-1β in microglia from the cervical spinal cord immediately after 8 hours of IH and for at least 16 hours post-IH.…”

“…To gain a greater understanding of the molecular pathway associated with pLTF inhibition, we targeted p38 MAPK, a common downstream signaling molecule activated by inflammation and which promotes further inflammation (Kumar et al, 2003). Like ketoprofen, spinal inhibition of p38 MAPK restored pLTF after IH-1, but suggested spinal inflammation is key to undermining pLTF (Huxtable et al, 2015). Since gene expression data suggests involvement of both microglia and other cell types, we used immunohistochemistry to begin to visualize changes in different cell types.…”

“…Thus, IH-1-induced inflammation activates p38 MAPK in phrenic motor neurons or nearby microglia to inhibit pLTF. While IH-1 elicits spinal inflammation and abolishes pLTF (Huxtable et al, 2015), seven days of IH (5 min hypoxia, 5 min normoxia, 12 hours) enhances pLTF and ventilatory LTF (Ling et al, 2001; McGuire et al, 2003). These differences may be due to the magnitude of inflammatory signaling or increased expression of trophic factors known to stimulate respiratory plasticity.…”

“…These differences may be due to the magnitude of inflammatory signaling or increased expression of trophic factors known to stimulate respiratory plasticity. For example, while one or three nights of IH induces inflammatory gene expression in spinal microglia (Huxtable et al, 2015; Smith et al, 2013), inflammatory gene expression in the spinal cord is not elevated after 14 consecutive nights of IH or after 4 weeks of thrice weekly IH (Peters et al, 2015; Smith et al, 2013). …”

Treatment options for apnoea of prematurity

“…Intermittent hypoxia leads to increased expression of inflammatory mediators in the central nervous system, which impairs respiratory motor plasticity in rat models 5. Rats exposed to intermittent hypoxia without concomitant inflammatory trigger demonstrated an increased response to hypoxia, whereas exposure to proinflammatory lipopolysaccharide (LPS) in neonatal rats delays and attenuates the carotid body response to hypoxia 6 7.…”