Intrathecally administered dynorphin-(1-17) modulates morphine-induced antinociception differently in morphine-naive and morphine-tolerant rats.

The intrathecal (i.t.) administration of morphine, but not of dynorphin-(1-17), dose dependently inhibited thermal nociceptive responses to radiant heat. In morphine-naive rats, i.t. dynorphin-(1-17) slightly antagonized morphine analgesia but potentiated it in morphine-tolerant rats. These data provide further evidence that dynorphin-(1-17) modulates the effects of exogenously administered opiates differently, depending upon whether the animals are opiate-naive or opiate-tolerant.