Investigation into nTS mu opioid receptor mechanisms contributing to fentanyl-induced cardiorespiratory depression - PROJECT SUMMARY This project aims to explore mechanisms involving mu opioid receptor (MOR) signaling within the nucleus of the solitary tract (nTS) in mediating fentanyl-induced cardiorespiratory depression. Over the past decade, synthetic opioids, primarily fentanyl, have been the leading cause of the surge in opioid-related deaths. Fentanyl presents a significant treatment challenge due to its rapid onset of effects, which makes timely intervention strategies difficult to implement. Numerous studies have examined opioid-MOR mechanisms in ventral brainstem respiratory networks as key factors in the development of opioid-induced respiratory depression (OIRD). The nTS, located in the dorsal brainstem, contains robust mu opioid receptor expression, including vagal afferent fibers that terminate in this region as well as postsynaptic expression on nTS neurons themselves. Despite this abundance of MOR in the nTS, the extent to which the nTS mediates OIRD has not been thoroughly investigated. Therefore, the goal of this proposal is to examine mechanisms by which opioids impact neuronal activity within the nTS, and how this relates to the onset, severity, and duration of OIRD. We will employ various complementary techniques to evaluate opioid-MOR mechanisms within the nTS. Aim 1 will utilize a newly generated transgenic MOR-Cre rat line, combined with chemogenetics, optogenetics, and fiber photometry, to evaluate presynaptic and postsynaptic mechanisms of fentanyl-MOR signaling in the nTS. Aim 2 will use two additional transgenic rat lines to evaluate how vagal afferent fibers mediate the activity of catecholaminergic and GABAergic nTS neurons, and how fentanyl impacts this vagal signaling to these nTS neuronal subpopulations. This aim will employ fiber photometry, chemogenetics and immunohistochemistry to examine these peripheral-central circuits in the context of OIRD. Aim 3 will utilize transgenic rat lines to determine how opioids directly impact the activity of catecholaminergic and GABAergic nTS neurons whether they are necessary for the onset, severity, or duration of OIRD. Additionally, this aim will utilize a transgenic mouse line in which MOR has been knocked out of noradrenergic cells. Experiments using these mice will determine the extent to which MOR-expressing noradrenergic cell groups, including in the nTS, contribute to the induction and severity of OIRD. This research will be supported by primary mentor Dr. Jose Moron-Concepcion at Washington University in St. Louis and co- mentor Dr. Jordan McCall at Washington University in St. Louis and the University of Health Sciences & Pharmacy in St. Louis. Additional external support will come from Dr. Erica Levitt at the University of Michigan and Dr. Kevin Cummings at the University of Missouri. By investigating opioid mechanisms in the nTS, the overarching goal of this research is to advance our understanding of how opioids induce cardiorespiratory depression, potentially by preventing the engagement of reflex responses that would normally be engaged to facilitate a return to cardiorespiratory homeostasis. Data generated during this project period seeks to contribute to the development of new therapeutic strategies for managing OIRD in individuals with opioid use disorders.
