Differential sensitivity to physical dependence on morphine and codeine in three inbred strains of mice

The purpose of this experiment is to investigate genetic differences in the development of physical dependence on morphine and codeine in inbred strains of mice, C57BL/6, C3H/He and DBA/2. Mice were treated with morphine- or codeine-admixed food (1, 2 and 3 mg/g of food) for 3 to 9 days. After the termination of drug treatment, the mice were given naloxone (5 mg/kg, s.c.). The incidences of jumping and teeth chattering by naloxone challenge in morphine- and codeine-treated C57BL/6 mice were much greater than those in C3H/He and DBA/2 mice. However, the incidences of other naloxone-precipitated withdrawal signs, such as ptosis and diarrhea, were not different among the three inbred strains of mice. These results indicate that genotype is an important determinant of the degree of most naloxone-precipitated withdrawal signs in morphine- and codeine-treated mice.