ABSTRACT Neural systems, including the system controlling breathing, exhibit plasticity, a change in future behavior based on prior experience. In anesthetized rats I will monitor phrenic nerve activity, serving as an electrical proxy for diaphragm muscle activation and respiratory motor output. Among the various forms of phrenic motor plasticity, phrenic long-term facilitation (pLTF) is extensively studied, which can be induced by repeated exposures to brief episodes of low oxygen, known as acute intermittent hypoxia (AIH). AIH has shown promise as a therapeutic approach for improving breathing and motor functions in individuals with spinal cord injuries or ALS. The induction of pLTF by AIH involves two distinct mechanisms: the Q and S pathways. The Q pathway requires the activation of carotid chemoafferents which project to brainstem raphe neurons inducing spinal serotonin (5-HT) release, and activation of phrenic motor neuron 5-HT2 receptors. In contrast, the S pathway relies on spinal tissue hypoxia, glial ATP/adenosine release, and activation of phrenic motor neuron A2A receptors. These pathways interact through "cross-talk inhibition," where the balance between Q and S pathways regulates AIH- induced pLTF. Notably, the complete abolition of plasticity occurs when both serotonin and adenosine-dependent mechanisms are equally activated. Therefore, shifting the balance away from equal activation of the Q and S pathways may enhance the induction of pLTF by AIH. In human studies, sustained hypercapnia or acute intermittent hypercapnic-hypoxia (AIHH) with isocapnic maintenance during recovery is necessary for long-term facilitation (LTF). Preliminary data from rat studies suggest that AIHH induces approximately double the pLTF compared to AIH alone. The enhanced pLTF induced by AIHH versus AIH is likely attributed to concurrent increase in Q pathway dominance and the simultaneous reduction of cross-talk inhibition from S pathway activation. Accordingly, this proposal aims to uncover the mechanism(s) behind the enhanced pLTF induced by AIHH compared to AIH. I hypothesize that AIHH enhances Q pathway dominance by increasing carotid chemoreceptor activation and amplifying serotonergic raphe neuron activity (Aim 1). I also hypothesize that AIHH alleviates S pathway constraints by increasing spinal tissue blood flow during hypoxic episodes, preventing a dramatic drop in spinal tissue PO2, and minimizing glial ATP/adenosine release (Aim 2). Participating in this research will provide me with new methods and insights, furthering our understanding of respiratory motor plasticity. It may also impact the design of clinical trials, considering the exploration of AIH as a treatment to enhance both respiratory and non-respiratory motor function in individuals with spinal cord injuries and ALS. Additionally, this research aligns with my broader career goal of transitioning to independence by eventually applying for a K99, allowing me to contribute significantl...