History and development of mixed opioid agonists, partial agonists and antagonists.

1 A brief history of the development of narcotic antagonists is outlined. 2 The clinical and pharmacological observations leading to the discovery of dualism of opiate receptors are reviewed. 3 An extension of this theory to a three-receptor model (mu, kappa and sigma) is required to rationalize the pharmacology of the antagonists analgesics cyclazocine, pentazocine and nalorphine. 4 The methodologies available for the study of compounds with dual agonist-antagonist activity are discussed in the light of the above receptor multiplicity, and data appertaining to the kappa-partial agonist, nalorphine and the mu-partial agonist buprenorphine are outlined.