Morphine as a discriminative cue in gerbils: drug generalization and antagonism.

Gerbils were trained in an electrified, T-shaped maze to discriminate between one of the three training doses of morphine (8, 16, or 32 mg/kg) and the nondrug condition. The rate of acquisition of the morphine discriminations was a function of dose, the high dose being the most rapidly discriminable condition. Dose generalization tests with morphine showed that the higher the training dose, the higher the ED50 value in producing 50% morphine-appropriate responding. Antagonism of the discriminable effects of morphine by naltrexone (dose range tested: 0.025-0.40 mg/kg) also was related to the training dose of morphine; i.e., the higher the training dose of morphine, the higher the corresponding ED50 value for blockade by naltrexone. A stereoisomeric requirement for morphine discrimination was evident since levorphanol, but not the analgesically inactive dextrophan, yielded morphine-appropriate responses when tested by substitution.