Strain difference in the effects of morphine on the rectal temperature and respiratory rate in male mice.

Effects of morphine on the rectal temperature and respiratory rate, and [3H]naloxone binding to brain membranes from seven brain regions were compared among six strains of male mice, including DBA/2N, C57BL/6N, BALB/c, C3H/HeN, A/J and ICR. The administration of 10 and 20 mg/kg doses of morphine HCl to these strains of mice decreased rectal temperature and respiratory rate. However, there was a significant strain difference in these two measures of the effect of morphine. The DBA/2N strain was the most sensitive in both measures of morphine action. The magnitude of hypothermia was positively correlated with respiratory depression among six strains of mice after the administration of 10 and 20 mg/kg morphine HCl, suggesting common mechanisms. Strain difference in naloxone binding in the brain regions could not explain that of the morphine responses, because there was no correlation between the intensity of morphine-induced hypothermia or respiratory depression and the regional [3H]naloxone binding for the mouse strains.