In cats implanted chronically with catheters in the lumbar epidural space, the pharmacology of the analgesia of 4 opiates, which varied widely in their physicochemical properties, was studied. Results revealed the following order of epidural analgesic potency as indicated by suppression of the spinally mediate skin twitch reflex: lofentanyl>morphine>l-methadone>meperidine>d-methadone. With the doses used in these experiments the duration of action was: morphine > lofentanyl >l-methadone = meperidine. In addition, antagonism of these effects by systemic naloxone and the development of tolerance after daily epidural administration of morphine were demonstrated. These data, jointly, indicate a specific receptor mechanism of action. We suggest that this animal model may be useful for the evaluation of new opiates and non-opiates to be given via the epidural route.
*Department of Anesthesiology, Presbyterian University Hospital, Pittsburgh, Pa. 15213 U.S.A.
**Section of Neurosurgical Research, Mayo Foundation, Rochester, Minn. 55901 U.S.A.
1 Please send reprint requests to Dr. Tony L. Yaksh.
(Received 4 March 1981, accepted 3 September 1981)