Discriminative stimulus effects of narcotics: evidence for multiple receptor-mediated actions.

The different pharmacological syndromes produced by morphine and related drugs in the chronic spinal dog led Martin and his colleagues (1,2) to suggest that these drugs exert their agonist actions 0y interacting with three distinct receptors (~,K, and e). Morphine was hypothesized to be an agonist for the p receptor, ketazocine (ketocyclazocine) was an agonist for the K receptor, and SKF-10,0q7 was an agonist for the ~ receptor. The effects of these three drugs in the chronic spinal dog were reversed by the narcotic antagonist, naltrexone, indicating that the effects of these drugs are narcotic agonist effects (I).