Adenosine 2A Receptor Inhibition Enhances Intermittent Hypoxia-Induced Diaphragm but Not Intercostal Long-Term Facilitation

Abstract

Acute intermittent hypoxia (AIH) elicits diaphragm (Dia) and second external intercostal (T2 EIC) long-term facilitation (LTF) in normal unanesthetized rats. Although AIH-induced phrenic LTF is serotonin dependent, adenosine constrained in anesthetized rats, this has not been tested in unanesthetized animals. Cervical (C2) spinal hemisection (C2HS) abolishes phrenic LTF because of loss of serotonergic inputs 2 weeks post-injury, but LTF returns 8 weeks post-injury. We tested three hypotheses in unanesthetized rats: (1) systemic adenosine 2aA (A2A) receptor inhibition with intraperitoneal (IP) KW6002 enhances Dia and T2 EIC LTF in normal rats; (2) Dia and T2 EIC LTF are expressed after chronic (8 weeks), but not acute (1 week) C2HS; and (3) KW6002 enhances Dia and T2 EIC LTF after chronic (not acute) C2HS. Electromyography radiotelemetry was used to record Dia and T2 EIC activity during normoxia (21% O 2 ), before and after AIH (10, 5-min 10.5% O 2 , 5-min intervals). In normal rats, KW6002 enhanced DiaLTF versus AIH alone (33.1 -4.6% vs. 22.1 -6.4% baseline, respectively; p < 0.001), but had no effect on T2 EIC LTF ( p > 0.05). Although Dia and T2 EIC LTF were not observed 2 weeks post-C2HS, LTF was observed in contralateral (uninjured) Dia and T2 EIC 8 weeks post-C2HS (18.7 -2.7% and 34.9 -4.9% baseline, respectively; p < 0.05), with variable ipsilateral expression. KW6002 had no significant effects on contralateral Dia ( p = 0.447) or T2 EIC LTF ( p = 0.796). We conclude that moderate AIH induces Dia and T2 EIC LTF after chronic, but not acute cervical spinal injuries. A single A2A receptor antagonist dose enhances AIH-induced Dia LTF in normal rats, but this effect is not significant in chronic (8 weeks) C2HS unanesthetized rats.

Enhanced recovery of breathing capacity from combined adenosine 2A receptor inhibition and daily acute intermittent hypoxia after chronic cervical spinal injury

“…Telemetry implantation and C2 cervical hemisection were performed under sterile conditions as described previously (Navarrete-Opazo et al, 2014, 2015). Rats were pre-medicated with subcutaneous buprenorphine (0.03 mg/kg), carprofen (Rimadyl, 5 mg/kg) and enrofloxacin (Baytril, 4 mg/kg); additional injections were made every 12 h for 2 days post-surgery to minimize post-operative pain and infection.…”

“…In normal rats, AIH-induced pLTF is serotonin-dependent (Bach and Mitchell, 1996; Baker-Herman and Mitchell, 2002), and is constrained by spinal adenosine 2A (A 2A ) receptor activation (Hoffman et al, 2010). Thus, A 2A receptor inhibition enhances AIH-induced phrenic and diaphragm LTF in normal rats (Navarrete-Opazo et al, 2014). In contrast, AIH-induced T 2 intercostal LTF is greater than that observed in the diaphragm (Navarrete-Opazo et al, 2014), but A2A receptor inhibition has minimal impact on this response.…”

“…Thus, A 2A receptor inhibition enhances AIH-induced phrenic and diaphragm LTF in normal rats (Navarrete-Opazo et al, 2014). In contrast, AIH-induced T 2 intercostal LTF is greater than that observed in the diaphragm (Navarrete-Opazo et al, 2014), but A2A receptor inhibition has minimal impact on this response.…”

“…When one week of daily AIH (dAIH) begins one week post-C2HS, breathing capacity and phrenic nerve activity are increased (Lovett-Barr et al, 2012) Although we originally hypothesized that A2A receptor inhibition would enhance IH-induced functional recovery of breathing capacity following C 2 HS, it actually blocked the effects of dAIH at this time (Navarrete-Opazo et al, 2014). In retrospect, this observation is consistent with reports that AIH fails to elicit pLTF two weeks post-C2HS due to reduced serotonergic innervation in the phrenic motor nucleus (Golder and Mitchell, 2005).…”

Swallow and breathing coordination following suprahyoid muscle injury.

“…Although both our pre-and post-measures were treated equally, there is potential influence due to the increased respiratory drive elicited (Navarrete-Opazo, Vinit, & Mitchell, 2014;Vinit, Keomani, Deramaudt, Bonay, & Petitjean, 2016). C2…”