PROJECT SUMMARY/ABSTRACT In the first funding period, we discovered that local mechanisms sense and respond to a reduction in synaptic inputs to phrenic motor neurons to elicit compensatory enhancement of phrenic motor output, a novel form of plasticity that we termed inactivity-induced phrenic motor facilitation (iPMF). We defined key cellular pathways that give rise to iPMF following prolonged reductions in phrenic neural activity and found that these same mechanisms do not give rise to iPMF following intermittent reductions in phrenic neural activity. Since many clinical disorders are characterized by recurrent, brief reductions in respiratory neural activity, our specific goal in the present project period is to investigate cellular mechanisms that give rise to iPMF following intermittent phrenic neural hypoactivity and begin studies investigating the role for iPMF in the control of breathing. Our working model is that intermittent reductions in phrenic synaptic inputs stimulates retinoic acid synthesis in the phrenic motor nucleus, which activates RARα receptors in phrenic motor neurons to increase activity of the atypical PKC isoform PKCζ and give rise to iPMF (Aim 1). We hypothesize that spinal mechanisms that give rise to iPMF result in a lowering of the CO2 threshold for phrenic inspiratory activity (Aim 2). We propose that induction of a distinct form of plasticity known as phrenic long-term facilitation (pLTF) by concurrent exposure to hypoxia undermines iPMF due to an NMDA receptor-mediated constraint of the cellular pathways giving rise to iPMF (Aim 3). Our fundamental hypothesis is that iPMF is a compensatory mechanism that detects and corrects reductions in phrenic motor output, thereby preventing apneas and hypopneas (Aim 4). A detailed understanding of cellular cascades giving rise to and constraining iPMF is essential to understand the physiological role of this highly novel form of plasticity, and—importantly—to identify promising therapeutic targets for pharmacological interventions to treat respiratory control disorders characterized by recurrent disruptions in respiratory neural activity, such as central sleep apnea.