Hypoxia, sleep and respiration in relation to opioids in developing swine

To test the role of mu and delta opioid systems in neonates during hypoxia, a total of sixteen, 4-11 (n = 7) and 26-33-day-old piglets (n = 9) were instrumented aseptically for assessment of sleep/wake states (S/W), electromyographic activities of the diaphragm and posterior cricoarytenoid muscles (EMGdi, EMG-pca, respectively), heart rate, and arterial pressures, pH and gas tensions. During daily sessions for 5 consecutive days, the piglets inhaled 10% O2/90% N2 for 10 min twice per session, first before any drug, then after either naltrexone (2 mg.kg-1 i.v.), a predominantly mu opioid antagonist, or naltrindole (4 mg.kg-1 i.v.), a specific delta opioid antagonist. During hypoxia, young, in contrast to older piglets, spent more time asleep, and increased sleep during the second half of the hypoxic exposure before, but not after each antagonist. They also exhibited, overall, higher breathing frequency, and lower slope, amplitude, area and initial area of EMGdi and EMGpca activity than older piglets. Naltrindole stimulated EMGpca activity in both age groups, and naltrexone increased the breathing frequency and slope of EMGdi in the older group. We conclude that hypoxia enhances the activation of central mu and delta opioid systems which influence S/W and respiration.