Developmental responses to opioids reveals a lack of effect on stress-induced corticosterone levels in neonatal rats.

The neonate has an unusual capacity for survival and the possibility exists that mechanisms for controlling stress responses may differ in the developing animal. In adults both endogenous and exogenous opioids can modulate the corticosterone responses to stress. We have studied this effect in neonatal rats and found that opioid modulation is absent in early postnatal development. Neonatal rats of either sex were injected with morphine (5-50 mg kg-1), fentanyl (10-100 micrograms kg-1), buprenorphine (0.1-30 mg kg-1) or naloxone (0.1-10 mg kg-1) and plasma corticosterone measured fluorimetrically 15 or 20 min later. In addition naloxone reversibility studies (1 mg kg-1, co-administered) were carried out for the opioid agonists. In adult rats, elevations in plasma corticosterone caused by injection stress were potentiated by morphine, fentanyl and buprenorphine. In neonates, though injection stress-induced rises in plasma corticosterone were absent at 10 days, elevations were observed at 21 days and later. However, significant potentiation of this corticosterone response by fentanyl was absent at 21 days and at later ages (30 and 40 days) for morphine and buprenorphine. The potentiating effect of all three agonists did not become fully effective until day 45. In addition, in animals acclimatized to injection stress by 7 day injection pretreatment, fentanyl did not significantly alter corticosterone levels in 30 day old neonates. High doses of naloxone (10 mg kg-1) significantly increased the corticosterone response to injection stress in adult rats but this effect was absent in 30 day old animals. A dose of naloxone (I mg kg-') which had no significant effect on the corticosterone response inhibited the effects of morphine, fentanyl and buprenorphine in 45 day old and adult rats. 5 This late development of opioid action is unusual in comparison with the maturation of endogenous peptides, receptors and antinociceptive responses and suggests that alternative mechanisms may be involved in stress-control in the neonate.