Voluntary consumption of morphine in 15 inbred mouse strains

To determine genetic differences in voluntary morphine consumption, 15 commonly used inbred strains of mice were given ad libitum two-bottle choice between saccharin alone or saccharin/morphine in one bottle and water in the other bottle. Subsequently, the saccharin was gradually reduced to zero, leaving only morphine. Independent groups of mice of the same strains were exposed to quinine in a parallel manner to control for the bitter alkaloid taste of morphine. Of the 15 strains, the C57BL/6J strain showed the highest consumption of morphine, both with or without saccharin and greatest consumption of morphine relative to quinine; it also showed only a slight decline in fluid consumption when morphine was added to the saccharin bottle. In marked contrast, the SWR/J strain showed the least consumption of morphine by the same criteria, followed closely by the AKR/J, CE/J, DBA/2J and SJL/J strains. The strain differences for all the morphine drinking measures exceeded an order of magnitude. Strain-specific voluntary morphine/saccharin consumption was not significantly correlated with saccharin consumption alone, but was highly correlated with morphine consumption alone. The results show that these behaviors are under an unusually large degree of genetic determination, and some of the largest strain differences remained essentially the same regardless of whether saccharin was present, or whether quinine was used as a control tastant.