Tonic nociception in neonatal rats.

The issues of whether infants detect noxious stimuli and whether their nociceptive responses are suppressed by analgesics has been the focus of considerable controversy. Therefore, to more completely assess the nociceptive responses of neonatal rat pups to tonic pain, we tested 3-day-old rat pups using the formalin test. The responses of the young pups to formalin-produced injury were similar to those observed in adult rats, both behaviorally and in terms of their responsivity to morphine-induced antinociception. These results provide the first clear-cut evidence of integrated tonic pain responses in the neonate.