Mild inflammation impairs acute intermittent hypoxia-induced phrenic long-term facilitation by a spinal adenosine-dependent mechanism.

Inflammation undermines neuroplasticity, including serotonin-dependent phrenic long-term facilitation (pLTF) following moderate acute intermittent hypoxia (mAIH: 3, 5-min episodes, arterial Po2: 40-50 mmHg; 5-min intervals). Mild inflammation elicited by a low dose of the TLR-4 receptor agonist, lipopolysaccharide (LPS; 100 µg/kg, ip), abolishes mAIH-induced pLTF by unknown mechanisms. In the central nervous system, neuroinflammation primes glia, triggering ATP release and extracellular adenosine accumulation. As spinal adenosine 2 A (A2A) receptor activation impairs mAIH-induced pLTF, we hypothesized that spinal adenosine accumulation and A2A receptor activation are necessary in the mechanism whereby LPS impairs pLTF. We report that 24 h after LPS injection in adult male Sprague Dawley rats: 1) adenosine levels increase in ventral spinal segments containing the phrenic motor nucleus (C3-C5; P = 0.010; n = 7/group) and 2) cervical spinal A2A receptor inhibition (MSX-3, 10 µM, 12 µL intrathecal) rescues mAIH-induced pLTF. In LPS vehicle-treated rats (saline, ip), MSX-3 enhanced pLTF versus controls (LPS: 110 ± 16% baseline; controls: 53 ± 6%; P = 0.002; n = 6/group). In LPS-treated rats, pLTF was abolished as expected (4 ± 6% baseline; n = 6), but intrathecal MSX-3 restored pLTF to levels equivalent to MSX-3-treated control rats (120 ± 14% baseline; P < 0.001; n = 6; vs. LPS controls with MSX-3: P = 0.539). Thus, inflammation abolishes mAIH-induced pLTF by a mechanism that requires increased spinal adenosine levels and A2A receptor activation. As repetitive mAIH is emerging as a treatment to improve breathing and nonrespiratory movements in people with spinal cord injury or ALS, A2A inhibition may offset undermining effects of neuroinflammation associated with these neuromuscular disorders.NEW & NOTEWORTHY Mild inflammation undermines motor plasticity elicited by mAIH. In a model of mAIH-induced respiratory motor plasticity (phrenic long-term facilitation; pLTF), we report that inflammation induced by low-dose lipopolysaccharide undermines mAIH-induced pLTF by a mechanism requiring increased cervical spinal adenosine and adenosine 2 A receptor activation. This finding advances the understanding of mechanisms impairing neuroplasticity, potentially undermining the ability to compensate for the onset of lung/neural injury or to harness mAIH as a therapeutic modality.