Abstract
Fentanyl is widely used perioperatively and illicitly as a drug of abuse. As a potent μ-opioid receptor agonist, fentanyl canonically inhibits excitability through Gα(i/o) intracellular signalling pathways resulting in analgesia and respiratory depression. However, fentanyl also paradoxically activates respiratory muscles causing a potentially lethal effect termed wooden chest syndrome. Here we show that fentanyl, but not morphine, causes a persistent tonic component of diaphragmatic muscle activity. Voltage-clamp studies reveal that fentanyl directly blocks a subset of ether-à-go-go-class potassium (K(+)) channels. These channels are widely expressed in spinal motoneurons, including those innervating the diaphragm. A significant fraction of these motoneurons are excited by fentanyl, concomitant with blockade of K(+) currents. Taken together we identified a novel off-target mechanism for fentanyl action, independent of μ-opioid receptor activation. Our findings may inform the design of safer analgesics and generalize beyond the activation of motoneurons to other neuronal circuits implicated in fentanyl-related maladaptive behaviours. KEY POINTS: High doses of fentanyl can cause a lethal phenotype termed 'wooden chest syndrome' (WCS) resulting from tonic contractions of respiratory-associated musculature, precluding the ability to mechanically inflate the lungs. In vivo murine diaphragmatic electromyograms reveal a tonic component of muscle activity elicited by fentanyl, but not morphine. Fentanyl reversibly blocks a subset of ether-à-go-go (EAG)-class potassium channels (EAG/Kv10 and ERG/Kv11 subclasses) expressed in HEK293 cells. Computational docking of fentanyl into cryogenic electron microscopy structures of these potassium channels predicts a binding site beneath the K(+) selectivity filter. RT-PCR and RNA-scope in situ experiments reveal widespread expression of EAG/Kv10 (Kcnh1, Kcnh5) and ERG/Kv11 (Kcnh2, Kcnh6, Kcnh7) transcripts in cervical motoneurons, including phrenic motoneurons retrogradely labelled from the diaphragm. In vitro patch-clamp recordings from cervical spinal sections identifies a significant fraction of phrenic motoneurons (44%) electrically excited by fentanyl, concomitant with the blockade of a non-inactivating voltage-gated potassium current. Direct block of EAG potassium channels by fentanyl may contribute to WCS.