One- and two-receptor models of opioid drug action.

The history of models for opioid drug receptors is traced and two currently proposed models are compared, the first envisaging two rigid receptors, one for agonists and one for antagonists, while the second model postulates one single flexible receptor which in its native conformation fits opioid antagonists but is distorted when it binds to agonists. Important recent experiments are satisfactorily interpreted by both models but the flexible one-receptor model requires fewer assumptions and explains more facts.