Genotype and test experience determine responsiveness to morphine.

Initial responsiveness to morphine was studied in two inbred strains of mice, C57BL/6J and DBA/2J, and their F1 hybrid, using both a hot-plate analgesia test and a locomotor-activity test. Three dose levels of morphine were used, 0 mg/kg 5 mg/kg, and 15 mg/kg. The inclusion of the 0 mg/kg group revealed differences between the inbred strains in the effects of test experience. These data also led to some new conclusions about the differences in responsiveness to morphine between the strains studied. On both tests, the DBA mice showed no effect of morphine, the C57 mice showed large effects, and the F1 mice showed an intermediate effect.