Effect of naloxone on ventilation in newborn rabbits

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Abstract
Endorphins were isolated from amniotic fluid and cord blood of mammals. To determine if these agents influence ventilation after birth ventilation (.ovrhdot.VE), tidal volume, inspiratory time, and respiratory frequency (f) were measured in 19 rabbit pups before and after administration of naloxone (NLX), an endorphin antagonist. Tracheostomy and carotid artery cannulation were performed under light ether anesthesia. After 30-90 min for recovery the pups were placed in a body plethysmograph. Rectal temperature was kept at 37 .+-. 0.5.degree. C. After 15 min of control measurements the saline was infused, which had no respiratory effect. NLX (4 .mu.g/g) was then infused and measurements continued for 30 min. In 6 of 7 pups .ltoreq. 4 days old, .ovrhdot.VE increased to 140-180% of control values and remained elevated for the remainder of the study period. Increased .ovrhdot.VE was due solely to increased f. By contrast, only 1 of 12 pups .gtoreq. 5 days old responded in this fashion. This difference was significant (P < 0.005). Arterial blood gases were measured before and after NLX in 10 pups. In those pups who increased their ventilation after NLX, arterial CO2 tension fell and pH rose above pre-NLX values (P < 0.05 for both variables). Blood gases of the group whose ventilation was uneffected remained unchanged. Early in postnatal life endorphins probably modulate central respiratory drive in rabbits but that the agents become less important with maturation.