Disruptive effects of naloxone on development of morphine tolerance

In rats the development of one-trial tolerance to the analgesic actions of morphine is disrupted by the post-administration of naloxone at 5 min, 3 hrs, or 24 hrs. Naloxone injections alone 24 or 48 hrs prior to the analgesic test failed to counteract morphine induced analgesia. It is suggested that naloxone initiates long term biochemical changes that oppose those produced by morphine.