Ampakine CX717 Stimulates Diaphragm Activity following Mid‐cervical Spinal Contusion Injury

Abstract

Cervical spinal cord injuries result in significant respiratory compromise and there is a need to develop therapies to counter respiratory insufficiency in this patient population. Ampakines are positive allosteric modulators of AMPA receptors, and can stimulate breathing during states of hypoventilation, such as following opioid overdose or in neuromuscular disorders. Our laboratory has previously shown that acute treatment with ampakine CX717 can stimulate phrenic activity in spinally injured animals under urethane anesthesia. Recently, we extended these findings by showing that a low dose of CX717 was sufficient to enhance diaphragm electromyography (EMG) activity and tidal volume in unanaesthetized animals with a C2 hemisection injury. The next step in the translational pathway, and the goal of the current study, was to test the efficacy of ampakines in a model of contusion injury that is more similar to the injuries typically experienced by humans. The C4 contusion injury used in the current study results in direct phrenic motor neuron loss and significant damage of the spinal pathways innervating respiratory motor pools. This model is also associated with strong spontaneous recovery of diaphragm function, such that pronounced deficits are likely to be detected only in the initial week following injury. Animals were implanted with in‐dwelling bilateral diaphragm EMG electrodes and ventilation was measured using whole body plethysmography. At 4 and 14 days following a 150 kDy C4 unilateral contusion injury (C4Ct), male and female rats were given a single intravenous bolus of CX717 (5 mg/kg, n=4) or vehicle (2‐hydroxypropyl‐beta‐cyclodextrin; HPCD; n=4). At the 4‐day time point, infusion of CX717 increased peak diaphragm EMG (recorded ipsilateral to C4Ct) by 40‐50% when compared to vehicle group. Respiratory rate was also elevated in the CX717 group by 80% post‐infusion, indicating a likely supraspinal mechanism of the impact of ampakines. In the CX717 group, tidal volume was increased by 25±13% during infusion, 20±16% at 5 min post infusion and returned to baseline levels by 15 mins post CX717 infusion. There was no change in tidal volume in the vehicle group. During an acute respiratory challenge with hypoxia (10% O2), CX717 treated animals were able to increase diaphragm EMG (100±15%) to a greater extent as compared to vehicle (50 ± 23%, P=0.004). Similarly, during a combined hypoxia‐hypercapnia challenge (10% O2, 7% CO2), CX717 treated animals displayed an increased diaphragm EMG response (200±44%) compared to vehicle (100 ± 55%, P=0.0144). In contrast to the 4‐day data, there was no discernable impact of ampakines on baseline or challenged breathing at 14 days post C4Ct. This is perhaps not surprising because prior work establishes that in this injury model, robust compensation has occurred after 2 weeks, and deficits in diaphragm activation are difficult to detect. In summary, our data indicate that low dose, low impact ampakine treatment can increase diaphragm muscle output at an acute timepoint following mid‐cervical contusion type injuries. These studies pave the path for future experiments to test if ampakine treatment can be combined with rehabilitation techniques to further enhance the therapeutic effect of ampakines in this injury model.