Respiratory control during hypoxia in newborn rabbits: implied action of endorphins

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Abstract
The mechanism of ventilatory depression during hypoxia in the neonate is unknown. Since endorphins depress ventilation and their actions are antagonized by naloxone, the effect of naloxone on respiration during acute hypoxia was tested in newborn rabbits. In 27 tracheotomized, unanesthetized pups, 1-15 days old, ventilation (.ovrhdot.VE) was measured in a body plethysmograph. At all ages inhalation of 5% O2 initially increased .ovrhdot.VE; thereafter .ovrhdot.VE became depressed in association with a decrease in CO2 elminiation (.ovrhdot.VCO2). The time constant of .ovrhdot.VE depression increased with age. During ventilatory and .ovrhdot.VCO2 depression, slaine infusion had no effect. Infusion of naloxone (4 .mu.g/g body wt) abruptly stimulated .ovrhdot.VE and .ovrhdot.VCO2. Since naloxone acts by competitive blockade of opiate (endorphin) rectors, these data suggest the following: depression in respiration and metabolism during hypoxia is related to the action of endorphins; the degree of endorphin influence decreases with age; and naloxone is effective in reversing hypoxic respiratory depression provided apnea is not established.