Ampakine Pretreatment Enables One Episode of Hypoxia to Produce Phrenic Motor Facilitation with No Added Benefit of Additional Hypoxic Episodes

Abstract

Excitatory glutamatergic synaptic transmission acting via the α-amino-3-hydro-5-methyl-4-isoxazolepropionate acid receptors (AMPARs) is critical for respiratory rhythmogenesis as well as activation of respiratory motoneurons. A synthetic class of drugs called ampakines can stimulate breathing via positive allosteric modulation of AMPARs which enhances glutamatergic currents. Our laboratory has reported that anesthetized mice pretreated with ampakine CX717 demonstrate enhanced long-term facilitation (LTF) of hypoglossal motor output following repeated bouts of hypoxia. More recently, we reported that ampakine treatment prior to a single bout of hypoxia can produce sustained facilitation of phrenic motor output in anesthetized rats. In the current study, we tested the hypothesis that ampakine pretreatment would enhance the magnitude of phrenic LTF following multiple bouts of hypoxia. Phrenic nerve output, arterial blood pressure and end-tidal CO2 were recorded from urethane-anesthetized, mechanically ventilated and vagotomized adult male Sprague-Dawley rats. Phrenic burst amplitude recorded during the experiments was expressed relative to baseline (pre-treatment) activity (%BL). First, we verified the impact of a single injection of intravenous (iv) dose of ampakine (CX717, 15mg/kg) compared to vehicle solution (10% HPCD). CX717 caused a small and transient increase in phrenic burst amplitude and frequency, confirming previous reports by our group. Phrenic burst amplitude showed a transient increase (70±48%BL after 2 min) that gradually declined and was not different than baseline after 60 min (2±32%BL, n=4). Injection of the HPCD vehicle caused no discernable change in phrenic activity (2 min: 0±15%BL, 60 min: 3±5%Bl, n=4). Second, we investigated if ampakine treatment prior to acute intermittent hypoxia (CX717 + 3 five-minute bouts of hypoxia) would produce a more robust phrenic LTF when compared to ampakine followed by single bout of hypoxia (CX717 + 1 five-minute bout of hypoxia). Similar to our prior report, CX717 + 1 five-minute bout of hypoxia causes a sustained increase in phrenic burst amplitude (60 min: 96±61% Bl, n=8). However, contrary to our hypothesis, little phrenic LTF was observed in the CX717 + 3X hypoxia group (60 min: 24±29%Bl, n=8). Lastly, in a separate series of experiments, we verified the impact of 1 vs. 3 episodes of hypoxia on phrenic motor output, but in the absence of ampakine. Phrenic LTF was present after 3 five-minute bouts of hypoxia (60 min: 88±26%BL, n=3) but not 1 five-minute bout of hypoxia (60 min: 4±1%BL, n=3). These studies of anesthetized and spinal-intact rats indicate that phrenic LTF is not enhanced when ampakine pretreatment is followed by multiple bouts of hypoxia. Rather, the combination of ampakine and a single brief hypoxic episode appears to be ideal for producing sustained increase in phrenic motor output. These findings could have implications for hypoxia-based neurorehabilitation strategies, a method which is gaining traction in spinal cord injury rehabilitation. A single dose of ampakine may serve to “prime” the nervous system such that fewer exposures to hypoxia are required to produce sustained neuroplastic changes.