Ontogeny of respiratory sensitivity and tolerance to the mu-opioid agonist fentanyl in rat.

Whereas developmental changes in analgesic sensitivity and tolerance to the mu-opioid agonist fentanyl have been reported, knowledge of respiratory responses to that drug is lacking. Using 7- and 14-day-old (P7, P14) and adult conscious rats, we first established, using whole body plethysmography, the fentanyl dose that decreased minute ventilation by 50% (ED50) at each age. ED50 increased with postnatal age (40, 60 and 120 microg/kg sc, respectively), indicating a high sensitivity to fentanyl in the youngest rats that decreased with maturation.