Genetic analysis of hypothermia induced by morphine in two strains of inbred mice.

The degree of hypothermia elicited by morphine was greater in DBA/2 than C57BL/6 strain mice of both sexes. Hypothermia elicited by morphine was antagonized by naloxone in both strains of mice, suggesting the involvement of opioid receptors. To examine the role of genetic factors in the strain difference of morphine-induced hypothermia, the effect of morphine on changes in the rectal temperature was studied in the 6 generations of male mice, including the 2 inbred strains, P1 (DBA/2) and P2 (C57BL/6), their F1 and F2 hybrids, and 2 backcrosses, B1 (F1 X P1) and B2 (F1 X P2). The order of mean temperature decrease determined 40 min after 20 mg/kg morphine injection was P1 greater than B1 greater than F1 = F2 greater than B2 greater than P2. There was no maternal effect on the morphine responses of the F1 generation. Biometrical analysis revealed that DBA/2 (P1) is partially dominant over C57BL/6 (P2) and contribution of polygenes was suggested.