Multiple opiate receptor affinities of kappa and agonist/antagonist analgesics: in vivo assessment.

The affinities of kappa and agonist/antagonist (Ag/Ant) analgesics for mu and delta opiate receptors were examined in vivo in the rat. In the case of kappa agonists, these agents appear to be mu and delta antagonists in vivo. The mu antagonist activity appears to involve a specific isoreceptor population, namely mu-2 receptors. With Ag/Ant analgesics, a more complex pharmacology is evident such that at mu and delta receptor populations these agents can exhibit pure Ag, pure Ant or a combination of Ag and Ant actions. These activities vary with the neuronal localization of the receptor population being examined. In addition, complex species differences are evident with Ag/Ant actions.