Opioid inhibition of projection‐specific, respiratory‐related pontomedullary neurons

Opioids depress breathing by inhibition of inter‐connected respiratory nuclei in the pons and medulla. We have previously shown that mu opioid receptor (MOR) agonists hyperpolarize a population of neurons in the dorsolateral pons due to activation of G protein‐mediated inwardly rectifying potassium conductance. However, we do not know the projection target of MOR‐expressing, dorsolateral pontine neurons, if they project at all. Here, we show that MOR‐expressing, dorsolateral pontine neurons project to core respiratory nuclei in the ventrolateral medulla, identifying projection‐related heterogeneity of dorsolateral pontine neurons vulnerable to opioid inhibition. First, retrograde DIO‐eGFP injections into the ventrolateral medulla of oprm1‐cre mice showed that dorsolateral pontine neurons project to the Bötzinger complex (BötC), pre‐Bötzinger complex (preBötC), and rostral ventral respiratory group (rVRG), and are distinct from calcitonin gene‐related peptide (CGRP)‐expressing parabrachial neurons. Second, we performed whole‐cell, voltage‐clamp recordings from dorsolateral pontine Kölliker‐Fuse neurons that were retrogradely labeled by fluorescent bead injections into the ventrolateral medulla of wild‐type mice. MOR‐mediated outward currents were identified in dorsolateral pontine neurons that project to the BötC, preBötC, and rVRG, confirming that these projection neurons are functionally inhibited by MORs and vulnerable to opioid inhibition. Together, these approaches demonstrate that MOR‐expressing, dorsolateral pontine neurons project to medullary respiratory nuclei and further unravel mechanisms of opioid‐induced respiratory depression.