Patients with sleep apnea (SA) are highly sensitive to opioid-induced respiratory depression (OIRD) by fentanyl, which we recently showed is recapitulated in rodents exposed to chronic intermittent hypercapnic hypoxia (CIHH) to model SA. Given that endogenous opioids are elevated in cerebrospinal fluid of SA patients, OIRD hypersensitivity might reflect prevailing endogenous opioid inhibition of the respiratory network that leaves patients highly vulnerable to fentanyl OIRD. Considering that systemic naloxone increases circulating oxytocin and intranasal oxytocin improves breathing stability in SA patients, we reasoned that oxytocin might contribute to naloxone reversal of OIRD. We reported that systemic oxytocin (5-10 nmol/kg, iv), like naloxone (1 mg/kg, iv), effectively prevents and reverses fentanyl OIRD. Given that the ventilatory defense response to hypoxia depends on the hypothalamic paraventricular nucleus (PVN), we sought to determine if ongoing PVN inhibition by endogenous opioids contributes to SA related OIRD hypersensitivity. In normoxic rats (n=3), bilateral PVN blockade of mu-opioid receptors with naloxone (1.25 nmol/side) had no effect on phrenic nerve activity (neural inspiration). By contrast, the same treatment in CIHH (8% CO 2 , 8% O 2 , 8 h/day, 7 days) exposed rats (n=5) promptly increased PNA burst amplitude, consistent with tonic suppression of breathing by PVN endogenous opioids. Remarkably, in the normoxic group PVN naloxone effectively prevented fentanyl (60 µg/kg, iv) OIRD. Furthermore, systemic blockade of oxytocin receptors with atosiban (40 nmol/kg, iv) prevented the OIRD protection afforded by PVN naloxone. By contrast, in the CIHH group fentanyl OIRD was preserved after PVN naloxone. These findings suggest that endogenous opioids suppress PVN output signals, likely involving oxytocin, that protect against OIRD. CIHH/SA appears to alter respiratory network function in a manner that abrogates protection afforded by PVN output (i.e., oxytocin). Next, we tested whether CIHH triggers neurochemical plasticity that reduces the number of oxytocin-positive PVN neurons as a possible means of abrogating PVN driven OIRD protection. Oxytocin-Cre x ZsGreen mice were exposed to sham/CIHH and PVN sections were co-immunostained for oxytocin neurophysin-1 (red). Oxy-Cre x ZsGreen mice exhibited high labeling efficiency with nearly all PVN oxytocin neurons showing green and red fluorescence. Analysis revealed that CIHH did not change the number of PVN oxytocin neurons (sham = 812.5 ± 32 cells/section (n=10), CIHH = 800.8 ± 29 cells/section (n=9); p = 0.66). Collectively, findings suggest that endogenous opioids in the PVN suppress oxytocin output that has the capacity to protect against fentanyl OIRD, possibly by promoting a more robust ventilatory defense response. CIHH abrogates PVN protection by inducing functional changes within the PVN, the pontomedullary respiratory network or both. Diminished PVN-mediated OIRD protection by CIHH does not reflect neurochemical plasticity leading to loss of oxytocin neurons. 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.