Analgesic, locomotor and lethal effects of morphine in the mouse: strain comparisons

We have compared the analgesic, locomotor stimulatory and lethal effects of morphine in two strains of mice, C57BL/6BY and CXBK. The CXBK strain is known to be deficient in central opioid binding sites and to be less sensitive than the C57 strain to certain effects of morphine and endogenous opioids. We found that the CXBK strain was less sensitive than the C57s to the analgesic and locomotor effects of morphine, but did not significantly differ in regard to morphine's lethal effect. The strain differences in sensitivity to the analgesic and locomotor effects were not uniform. The CXBK strain was much less sensitive than the C57 strain to the analgesic effect but only moderately less sensitive to the locomotor stimulatory effect. These differences may relate to previously demonstrated strain differences in the amounts of mu 1 and mu 2 opioid binding in central nervous system areas thought to mediate these behaviors.