Sleep apnea (SA) patients are hypersensitive to opioid-induced respiratory depression (OIRD), which may be linked to reports of increased endogenous opioids in their blood and cerebrospinal fluid. Interestingly, systemic naloxone, which is the primary treatment for OIRD, increases oxytocin in the blood and cerebrospinal fluid. We recently reported that, like naloxone, systemic oxytocin can prevent and reverse fentanyl OIRD. Collectively, available evidence seems consistent with the possibility that an oxytocin signaling deficiency in SA, perhaps brought on by heightened endogenous opioids, might contribute to OIRD hypersensitivity. The pontine parabrachial/Kölliker-Fuse complex (PBN/KF), a major hypercapnia–hypoxia–responsive respiratory hub implicated in OIRD, receives dense monosynaptic input from PVN oxytocin neurons. Consistent with this circuits involvement in OIRD, our preliminary findings indicate that photoactivation (473 nm) of PVN-PBN/KF oxytocin neurons (10 ms pulses, 20 Hz, 5 s On/1 s Off, 3 min) reverses OIRD by systemic fentanyl (20 µg/kg, iv), Together with literature evidence, this finding suggests that impaired PVN-to-PBN/KF oxytocin transmission - perhaps reflecting suppression by endogenous opioids - contributes to fentanyl OIRD hypersensitivity in SA. To gain insight into this possibility, we examined neuronal adaptations to CIHH among PVN oxytocin neurons using whole-cell current-clamp recordings in brain slices from oxytocin-Cre x ZsGreen transgenic mice exposed to our 7-day CIHH protocol, modeling SA. Data were compared to sham-exposed mice. Magnocellular oxytocin neurons showed increased excitability compared to sham controls together with faster action potential rise (sham = 2.2 ms, n = 9; CIHH = 1.8 ms, n = 14; p = 0.005) and decay (sham = 4.5 ms, n = 9; CIHH = 3.4 ms, n = 15; p = 0.01) times. Unexpectedly, hyperexcitability was accompanied by a pronounced after-hyperpolarization potential (AHP) in CIHH (–11 mV, n = 17) that was largely absent from the sham group (n=10) (p = 0.0002). CIHH also reduced presumptive HCN channel-mediated sag (sham = 26 mV; 6/12 neurons; CIHH = 11 mV; 8/19 neurons; p = 0.05). By contrast, caudal PVN parvocellular oxytocin neurons - including those projecting to PBN/KF - showed reduced excitability with no changes in their action potential waveform. Their HCN sag was diminished in 50% of CIHH neurons (4/8), whereas all sham neurons (5/5) showed substantial sag to –50 pA current injection. Because magnocellular neurons can release oxytocin from their dendrites to influence parvocellular neurons, magnocellular neuron plasticity might increase extracellular PVN oxytocin, perhaps partially compensating for inhibitory actions of endogenous opioids that appear to suppress parvocellular oxytocin output to the respiratory network, including the PBN/KF. Findings so far suggest that CIHH induces cell-type-specific dysfunction of PVN oxytocin neurons that may participate in deficit oxytocinergic drive to respiratory network targets, contributing to OIRD hypersensitivity in CIHH/SA. Support DA060239 (GMT) This abstract was presented at the American Physiology Summit 2026 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.
Al-Yaari et al. (Fri,) studied this question.