Opioid sensitivity of projection‐specific Kölliker‐Fuse neurons

Abstract

Opioid‐induced respiratory depression is the primary cause of death in individuals who overdose on opioids. Respiration is controlled by a highly interconnected neural network located in the pons and medulla. The Kölliker‐Fuse nucleus (KF), located in the dorsolateral pons, is an essential part of this network and projects to core respiratory nuclei in the ventrolateral medulla. Opioids injections into the KF disturb respiratory rate, volume, and pattern. Deletion of opioid receptors from KF neurons attenuates morphine‐induced respiratory depression. These studies suggest that the KF is a critical site in opioid‐induced respiratory depression. Mu opioid receptor agonists hyperpolarize a population of neurons in the KF due to activation of G protein‐mediated inwardly rectifying conductance. However, we do not know the projection target of the opioid‐sensitive KF neurons. Given the extensive projections from the KF to medullary respiratory rhythm generators, we hypothesize that opioid‐sensitive KF neurons project to respiratory rhythm generators. Fluorescent beads (FluoSpheres 580/605, ThermoFisher) were injected into the Bötzinger complex (BötC), pre‐Bötzinger complex (preBötC), and rostral ventral respiratory group (rVRG) of mice. Whole‐cell voltage‐clamp recordings were made from retrograde‐labeled KF neurons contained in brain slices. Mu opioid receptor‐mediated outward currents were identified in KF neurons that project to the preBötC, and rVRG, but not in KF neurons that project to the BötC. Opioid‐sensitive connections between the KF and respiratory rhythm generators may be a possible mechanism by which opioids reduce respiratory rate.Support or Funding InformationR00, DA038069