Genetically determined differences in the effects of morphine on mice.

We have compared a variety of well known behavioral actions of morphine across four strains of laboratroy mice. Thes measures included locomotor activation, analgesia, tolerance, dependence and drug-seeking. We found that analgesia and drug-seeking did not vary greatly among strains, although some statistically significant differences were found. The interstrain rank ordering for locomotor activity, tolerance and dependence was different for each of these measures. This suggests that each of these effects of morphine may depend upon separate mechanisms, perhaps with each under independent genetic control. Other experiments were carried out to determine whether or not there were differences in the in vivo brain uptake of an opiate or the in vitro binding of naloxone to 'stereospecific receptors'. No significant interstrain differences could be found.