[Motility effects of methamphetamine in morphine-tolerant mice by the DAP method]

We reported that the reverse tolerance to the effect of morphine on ambulatory activity did not develop in mice treated with morphine-admixed food. In this study, effects of repeated administration of methamphetamine on ambulatory activity were studied in mice treated with morphine-admixed food. The ambulatory activity was determined by the tilting cage method. Methamphetamine injection (1 mg/kg, s.c.) was repeated at 3 approximately 4 day intervals. The ambulatory activity was enhanced progressively when methamphetamine was repeatedly given in mice. However, the enhancing effect was not observed in mice treated with morphine-admixed food (1 mg/g food). Moreover, in a single injection experiment, the effect of the combination of methamphetamine and morphine on ambulatory activity was more potent than that of each drug. These results suggested that the reverse tolerance to methamphetamine did not develop under the condition of exposure to morphine by the DAF (drug-admixed food) method, and the developmental mechanism of reverse tolerance to methamphetamine might be similar to that of morphine.