Ampakine‐Hypoxia Induced Phrenic Motor Facilitation is Not Prevented by Intrathecal Methysergide

Abstract

Hypoxia‐induced sustained increases in phrenic nerve burst amplitude lasting beyond the hypoxic stimulus are generally accepted to require repeated bouts of hypoxia (i.e., acute intermittent hypoxia, AIH). However, our laboratory recently reported that pretreatment with a drug (ampakine) which acts as a positive allosteric modulator of AMPA receptors lowered the number of hypoxic exposures required to trigger sustained phrenic motor facilitation. Our working hypothesis for the current study was that the combination of ampakine and hypoxia activates the same serotonin (5‐HT) dependent cellular mechanisms that are well established for AIH‐induced phrenic long‐term facilitation following repeated bouts of moderate hypoxia. Phrenic nerve output was recorded from vagotomized, mechanically ventilated, adult male Sprague‐Dawley rats under urethane‐anesthesia. A broad‐spectrum 5‐HT receptor antagonist (methysergide, 20mM, dissolved in DMSO) was delivered intrathecally over the C4 spinal dorsal surface. At least 15 min of stable phrenic burst amplitude was recorded post methysergide delivery. After a stable baseline (BL) was established, intravenous (i.v.) ampakine (CX717, 15mg/kg) was followed by a single episode of hypoxia (arterial partial pressure of O2 = 42±9mmHg). Phrenic nerve bursting was recorded for 60 min after hypoxia. Initial results indicate that intrathecal methysergide does not prevent the ampakine‐hypoxia induced phrenic motor facilitation as evidenced by a significant increase in burst amplitude at 60 min (89±54% BL, n=7). To verify the efficacy of intrathecal drug delivery method, additional experiments were conducted. Specifically, we determined if intrathecal methysergide prevented the phrenic long‐term facilitation induced by AIH, as has been shown by several prior investigations. These experiments used the same preparation, and AIH consisted of three‐5 min episodes of hypoxia with 5‐min intervals of baseline O2conditions. Intrathecal methysergide blocked pLTF (60 min post‐AIH, phrenic burst amplitude = ‐2±31% BL, n=3), but intrathecal delivery of the DMSO vehicle did not (60 min: 94±28% BL, n=3). Collectively, these results do not support our hypothesis that phrenic motor facilitation induced by the pairing of ampakine with a single bout of hypoxia is dependent on serotonin receptor activation.