Antinociceptive profiles of mu and kappa opioid agonists in a rat tooth pulp stimulation procedure

Abstract

The purpose of the study was to develop a tooth pulp stimulation procedure in the awake, freely moving rat and to then quantitatively assess the analgesic effects of compounds reported to act on mu and/or kappa receptors. The mu receptor agonists produce a biphasic (primary and secondary slope) dose-response curve (DRC) whereas kappa agonist and mixed agonist/antagonist analgesics produce single-slope DRCs. The primary slopes calculated from the mu agonist biphasic DRC are steeper than the slopes calculated for the other types of opioid analgesics. The rank order of analgesic potency for the mu agonist analgesics is oxymorphone greater than morphine = methadone greater than meperidine. The rank order analgesic potency for the kappa agonist analgesics is tifluadom greater than ethylketocyclazocine greater than U50488H and for the mixed agonist/antagonist analgesics, butorphanol greater than nalbuphine greater than pentazocine. The nonsteroidal anti-inflammatory drugs, aspirin and zomepirac, are also effective analgesics in this test procedure; but the DRC slopes for these compounds are lower than all opioid analgesics tested. The opioid antagonist naloxone produces no significant changes in threshold responses. Naloxone did reverse the threshold increases produced by morphine and ethylketocyclazocine but not aspirin. This study demonstrates that the electrical stimulation of the tooth pulp in the rat can be used as an assay for evaluation of opioid and nonopioid analgesics. When minimal effective dose values of each analgesic are plotted as a function of the clinical analgesic dose a high correlation is observed.