Morphine as a discriminative cue in gerbils: Drug generalization and antagonism

Abstract

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.