Morphine stimulates rapid, regular, deep, and sustained breathing efforts in fetal lambs

We gave morphine (1-4 mg) to 11 unanesthetized lamb fetuses at 123-141 days of gestation. After a delay of 28 +/- 7 min (mean +/- SE), the fetuses began forceful, frequent, regular, and sustained inspiratory efforts lasting continuously for 105 +/- 15 min and continuing intermittently for a total of 155 +/- 13 min. The induced breathing motions are more forceful, frequent, and regular than the naturally occurring efforts of the fetus and resemble neonatal breathing motions. The phenomenon is blocked by 0.5 mg of naloxone but not by 0.1 mg of atropine. We conclude that the mechanism of action involves the opiate receptor system. We speculate that endorphins may have a role in the regulation of breathing in the fetal and newborn lamb.