Trauma to the mid-cervical spinal cord causes disruption of bulbospinal pathways innervating the phrenic nucleus and direct loss of phrenic motoneurons, thereby resulting in diaphragm muscle paresis or paralysis. Respiratory complications arising from an inability to clear airways or breathe deeply are the leading cause of mortality and morbidity in the SCI patient population (Devivo, 2012; Fogarty and Sieck, 2020; National Spinal Cord Injury Statistical Center, 2016). Although most cervical SCIs are functionally incomplete with white and gray matter sparing, spontaneously occurring respiratory improvements following the initial trauma are largely sub-optimal. This underscores the need to develop therapies that enhance output of residual spinal respiratory neural circuits.

Ampakines are synthetic compounds that readily cross the blood brain barrier and enhance excitatory glutamatergic neurotransmission by slowing deactivation or attenuating desensitization of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (Arai and Kessler, 2007). Glutamate provides the main excitatory drive to respiratory motor neurons and acts in part via AMPA receptors (Chitravanshi and Sapru, 1996; McCrimmon et al., 1989; Rana et al., 2019). Thus, ampakines are a pharmacological tool to enhance AMPA receptor-mediated excitatory drive to the neural centers controlling respiratory motor output. Indeed, several studies have documented the efficacy of ampakines to stimulate breathing during hypoventilation, such as following opioid overdose (Oertel et al., 2010; Ren et al., 2009) or in neuromuscular disorders (ElMallah et al., 2015). Our laboratory has previously shown that acute intravenous (Wollman et al., 2020a) or intrathecal (Thakre et al., 2022) administration of ampakines can stimulate phrenic nerve activity in rats under urethane anesthesia. This effect is particularly prominent after cervical (C2) spinal hemilesion injury (Wollman et al., 2020b). Building upon those results, we recently observed that acute administration of ampakine CX717 or CX1739 can increase diaphragm electromyography (EMG) activity following a C2 spinal hemisection injury in awake, freely moving rats (Rana et al., 2021). The data from the C2 hemilesion model establish the proof of concept that low dose ampakine treatment can increase diaphragm activation after SCI (Rana et al., 2021; Wollman et al., 2020b). The next step in the translational pathway is to test the efficacy of ampakines in a model of cervical contusion injury that is more similar to injuries typically experienced by humans. In the current study, we employed a C4 contusion injury (C4Ct) model that causes motor neuron death, along with considerable disruption of descending bulbospinal pathways innervating respiratory motor pools (Golder et al., 2011; Khurram et al., 2019; Lane et al., 2012; Nicaise et al., 2012a; Nicaise et al., 2012b; Rana et al., 2017).

In the present study, animals were implanted with chronic in-dwelling bilateral diaphragm EMG electrodes to enable longitudinal, within-animal, quantitative measurements after C4Ct. This preparation was used to systematically assess the impact of acute ampakine treatment on diaphragm EMG activity during ventilatory behaviors requiring varying degrees of diaphragm muscle activation. We hypothesized that intravenous infusion of a low impact ampakine could stimulate diaphragm EMG activity and increase inspiratory tidal volume in awake unrestrained rats following unilateral C4Ct.