K⁺channel blockade by fentanyl: Implications for opioid overdose and Wooden Chest Syndrome

Fentanyl, a potent synthetic opioid, is widely known for its mu-opioid receptor (MOR)-mediated effects, including analgesia and respiratory depression. However, in addition to its action as a MOR agonist, we report a novel off-target action of fentanyl by direct blockade of specific subthreshold potassium (K + ) channels, disrupting neuronal excitability across multiple systems. This represents a critical and underexplored mechanism contributing to fentanyl's broad pathophysiological impact, with significant implications for opioid addiction, toxicity and overdose management. Using HEK293 cells expressing Eag and Erg subclasses of K + channels, we found that fentanyl (10 µM), but not morphine (10 mM) reversibly blocked these expressed K + currents by ~20-70% in a subclass-specific fashion. Eag/Erg K + channels are widely expressed in central neurons and may therefore affect numerous neuronal processes. Here, we investigated the consequences of this effect in mouse spinal motoneurons critical for respiratory control. In synaptically isolated phrenic motoneurons from in vitro C4-5 spinal slices, whole-cell patch-clamp recordings revealed that fentanyl attenuated non-inactivating outward K + currents and increased excitability in 44% of recorded motoneurons, consistent with an action on intrinsic membrane excitability by a fentanyl-dependent block of K + currents. These in vitro excitability changes were unaffected by naloxone, further suggesting a mechanism independent of MOR signaling. RT-PCR assays with isolated C4-5 ventral horn tissue and in situ hybridizations with RNAscope probes confirmed the expression of Eag/Erg transcripts in C4-5 motoneurons. By in vivo electromyography, we observed that fentanyl administration (500 µg/kg) in urethane-anesthetized mice induced a pronounced persistent tonic component of diaphragmatic activity throughout the respiratory cycle, while reducing respiratory rate by ~50%. Together these findings are consistent with high doses of fentanyl paradoxically increasing excitability in a subset of phrenic motoneurons through the blockade of Eag/Erg K + channels, leading to abnormal increased muscular tone and chest-wall rigidity, a hallmark of fentanyl-induced "Wooden Chest Syndrome", independent of MOR signaling. We hypothesize that this novel mechanism has potentially broad implications from altering general motor control exacerbating respiratory dysfunction, to possibly altering the excitability of neocortical reward circuitry mediating opioid addiction. Our findings establish direct K + channel blockade as an important mechanism contributing to fentanyl’s diverse and clinically significant impacts, offering new avenues for therapeutic interventions for opioid overdose. HL126523 (JMR); HL168211 (NJB) This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.