Unraveling the dynamic mechanisms underlying opioid respiratory depression

Seattle Children'S Hospital, Seattle WA

Investigators

Linked publications, trials & patents

Abstract

Project Summary Opioid overdose deaths continue to escalate with a large proportion caused by fentanyl overdoses. To explain why fentanyl is particularly fatal we propose the “triple-hit” hypothesis: Fentanyl targets three distinct but interacting mechanisms that lead to a fatal apnea: (1) Fentanyl causes wooden chest syndrome (WCS) by directly exciting motoneurons due to the inhibition of K+-channels; (2) Fentanyl activates tonic inhibitory neurons that slow rhythmogenesis within the ventral respiratory column (VRC) resulting in the opioid-induced respiratory depression (OIRD). As the network slows, it preserves the rhythmogenic properties of eupneic breathing, but WCS and OIRD together drive the network towards the third hit: (3) Hypoxic conditions. While under normoxic conditions, breathing depends on rotational dynamic interactions among inspiratory and expiratory neurons distributed along the entire VRC; In hypoxia this network reconfigures, becomes silent and dependent on the ballistic activation of inspiratory neurons to generate gasping and autoresuscitation. Although this grant focuses only on these mechanisms, we predict that the three distinct mechanisms will require distinct interventions to slow OIRD, reduce WCS and prevent or reverse the fatal apnea. These three distinct mechanisms are explored in three specific aims: Aim 1 characterizes the fentanyl effects on K+-currents expressed in HEK293 cells to unravel the mechanisms that activate neurons. In synaptically isolated motoneurons we will then characterize the fentanyl effects that lead to the activation of the diaphragmatic EMG. Aim 2 characterizes the opioid-induced transition from OIRD into the fatal apnea: We specifically test the hypothesis that under baseline conditions respiratory rhythmic activity is generated by a distributed network along the VRC. We propose that opioids activate inhibitory neurons in the parabrachial nucleus (PBN) and Kölliker-Fuse (KF) region which in turn inhibit neurons in the VRC and pons. These tonic inhibitory neurons play a significant role in depressing respiratory rhythmogenesis but they preserve the principle rhythmogenic mechanisms of the network. This hypothesis is tested with Neuropixel recordings in urethane-anesthetized, as well as alert freely behaving mice. Viral vectors injected into the medullary and pontine regions will allow us to optogenetically and chemogenetically manipulate inhibitory and excitatory neurons to assess their contributions to OIRD and fatal apnea. Aim 3 will use TRAP2 mice to explore the permanent induction of ASTEp1 Trap2 in neurons that are activated by opioids versus hypoxia. These mice will allow us to histologically characterize as well as optogenetically and chemogenetically manipulate neurons that are differentially activated by opioids versus hypoxia. Aims 1 to 3 complement each other to arrive at a thorough understanding of the network mechanisms that underlie opioid-induced respiratory depression, wooden chest syndrome and the fatal apnea.