Analgesic actions of mu- and kappa-opiate agonists in rats.

The antinociceptive profiles of mu- and kappa-opiate agonists were determined in a combined tail immersion plus tail pressure nociceptive test in rats. Combined analysis of potencies for pressure against heat noxia enabled differentiation between all mu- and kappa-agonists tested, thus confirming previous studies in other animal species and paralleling observations from studies on isolated tissue preparations. The distinct antinociceptive profiles produced by mu- and kappa-agonists provide further quantitative evidence that these agents are behaviourally different.