Differentiation of potent mu and kappa-opiate agonists using heat and pressure antinociceptive profiles and combined potency analysis.

The antinociceptive profiles of strong mu- and kappa-opiate agonists were determined in the tail clip, hot plate (55 degrees C) and tail immersion (48 degrees C and 55 degrees C) tests in mice. All mu- and kappa-agonists produced steep dose-response lines in all the tests. Using a combined analysis of potencies for pressure against heat noxia it was possible to differentiate between mu- and kappa-agonists and this paralleled findings in other laboratories using isolated tissue preparations. It was also concluded that qualitative rather than quantitative differences between noxia were responsible for the observed differentiation since potency differences for mu- and kappa-agonists between tests were maintained even after the intensity of heat and pressure noxia were equated. It was also proposed that use of the combined tail clip and immersion tests may be useful for rapid behavioural classification of novel synthetic mu- and kappa-opiate analgesics.