Background: Systemic opioids depress tidal volume, however, direct opioid effects on the phrenic motor nucleus (PhrMN) have not yet been studied in vivo. A recent in vitro study identified mu-opioid receptors on phrenic motoneurons, and fentanyl bath application increased or decreased neuronal discharge activity in neonatal mouse spinal cord slices (1). This is the first exploration of the effect of the mu-opioid receptor agonist D-Ala2, N-MePhe4, Gly-ol-enkephalin (DAMGO) on the entire PhrMN in an in vivo animal preparation. Methods: The study was approved by the local Animal Care Committee and conformed to NIH standards. Adult New Zealand White rabbits (2.5-3.5kg) of either sex were anesthetized, tracheotomized, ventilated, decerebrated and vagotomized. The cervical spinal cord was accessed through laminectomy of the spinous processes C2-C5. Bilateral phrenic nerve activity was recorded from the C5 rootlet, time averaged and used to calculate inspiratory and expiratory duration and peak phrenic activity (PPA). Using a multibarrel electrode with one recording and three injection barrels, the PhrMN was functionally identified through inspiratory neuron activity and increase in tidal volume with α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid injections (AMPA, 2.5mM, 70nl) at the depth of neuronal activity in 1-2mm rostro-caudal steps. DAMGO injections (200mM, 210nl) were performed in 2mm steps over the entire length of the previously identified PhrMN. The opioid antagonist naloxone (40 mg/kg) was given IV after the last DAMGO injection to confirm reversibility. Statistical comparison with RM ANOVA, data are mean±SD. Results: In 10 rabbits (2.9±0.2kg), we recorded inspiratory activity at 0.75-1mm lateral to midline and ~2-2.5mm deep over a length of maximally ~20mm. AMPA caused transient increases in PPA solely in the ipsilateral phrenic neurogram. Over an area of 8±2mm around the level of the C3 nerve root, AMPA also caused transient tonic activity. DAMGO injection decreased PPA to 63±15% (p< 0.001, n=7) while respiratory rate was not affected. The effect was reversed by IV naloxone. DAMGO effects were only observed in locations where AMPA caused tonic activity as confirmed by rostro-caudal and caudo-rostral injection order. Within this area, each DAMGO injection caused additional depression of the PPA, and the maximal effect was the cumulative effect of multiple injections. Conclusion: We found that the effects of AMPA and DAMGO injection on phrenic motor output varied throughout the PhrMN. While AMPA injections increased PPA over the entire length of the PhrMN, AMPA caused tonic activity only in a subarea around the C3 nerve root. DAMGO injection depressed PPA solely in the “tonic” subarea. It remains to be determined whether this is due to neuron density, differential neuronal activity at baseline or possibly differential distribution of mu-opioid receptors in the presynaptic neurons, interneurons, and motoneurons in the individual PhrMN subareas. (1) Wei et al., PMID 41110851 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.
Stucke et al. (2026) studied this question.