A rabbit tooth-pulp assay to determine ED50 values and duration of action of analgesics

The rabbit tooth-pulp assay is well established as a standard and reliable method to test for analgesic activity of drugs. Traditional methods to compare potencies of narcotic analgesics have been to establish ED50 values from rodent hot-plate and tail-flick tests. We describe a modification of the tooth-pulp assay with the use of a microcomputer to generate ED50 values based on "all or none" quantal responses and to evaluate durations of action, with the use of relatively few animals to test several drugs. The potencies of morphine, fentanyl, and alfentanil were established and compared to those from the mouse hot-plate assay. The rank order of potencies were the same and the absolute values were consistently lower. The ED50 value of pentazocine was determined with the tooth-pulp assay but could not be determined with the standard mouse hot-plate assay. The assay provides an additional, reliable, and sensitive method to generate ED50 values and to evaluate durations of action of narcotic analgesics.