Morphine sensitivity and tolerance: a genetic investigation in the mouse.

Sensitivity and tolerance to morphine were determined in 2 strains of mice, BALB/cBy and C57BL/6By, their reciprocal F1 hybrids and seven of their recombinant inbred strains. Sensitivity was established based on locomotor activity following the administration of saline, 10 or 20 mg/kg of morphine hydrochloride while tolerance was established according to the "hot plate" method following the single or repeated administration of saline, 5, 10, or 20 mg/kg of morphine hydrochloride. Results indicate that both sensitivity and tolerance to morphine are genotype-dependent and their inheritance is characterized by dominance or partical dominance. Further clarification of the genetic relationship of sensitivity, tolerance and analgesia to morphine must await analysis of the brain morphine-binding protein currently being conducted.