A COMPARISON OF THE ANTINOCICEPTIVE EFFECTS OF OPIOID AGONISTS IN NEONATAL AND ADULT-RATS IN PHASIC AND TONIC NOCICEPTIVE TESTS

Changes in the attitudes about neonatal pain and pain management have recently resulted in increases in the administration of opioids to neo nates. Little is known, however, about the relative potencies of the v arious opioid agonists employed, especially in comparison to adult res ponses. The first objective in the present study was to compare the an tinociceptive potency of four clinically relevant opioids in neonatal and adult rats. The second objective was to compare and contrast these agents in two different types of nociceptive tests: tonic (formalin-i nduced inflammation) and phasic (tail flick and hot plate). Our result s indicate that the opioid agonists morphine, meperidine, and fentanyl , and the mixed agonist buprenorphine were all effective antinocicepti ve agents in both neonates and adults in each of the three tests emplo yed, and that the relative potencies of these agents appeared to be si milar in neonates and adults. In general, the pups were more sensitive to the antinociceptive agents when tested in the phasic nociceptive t ests, and the drugs were more potent in the tonic test than either of the phasic tests.