The role of opioid receptors in morphine withdrawal in the infant rat

Exposure to opiates such as morphine can lead to psychological and physical dependence in both adult and infant humans. Infant rats experience opiate withdrawal behaviors that are qualitatively different from the withdrawal b ehaviors displayed by adult rats. In the adult. withdrawal is largely media ted by the mu -opioid receptor. We sought to understand more about what rol e each opioid receptor (mu, kappa, and delta) plays in the display of the p hysical withdrawal in the infant rat. Beginning on postnatal day 1, infant rats were injected with morphine sulfate twice a day for 6.5 days. On the a fternoon of the seventh day the infant rats were given an i.c. injection of a vehicle. the mu -opioid receptor antagonist CTOP, the kappa -opioid rece ptor antagonist nor-BNI, or the delta -opioid receptor antagonist naltrindo le. CTOP precipitated withdrawal behaviors in the 7-day-old rat in a dose-d ependent manner. Neither nor-BNI nor naltrindole induced any significant ch anges in the frequency of the withdrawal behaviors. These data suggest that in the infant rat control of certain behavioral withdrawal signs is modula ted primarily by the mu -opioid receptor, as is the case in the adult rat. (C) 2000 Elsevier Science B.V. All rights reserved.