Acute morphine dependence in mice selectively-bred for high and low analgesia.

Acute morphine dependence was compared in mice selectively-bred for high (HA) and low (LA) swim stress-induced analgesia and high (HAR) and low (LAR) levorphanol analgesia by counting the number of naloxone-precipitated jumps. Whereas LAR mice displayed greater acute morphine dependence than HAR mice, HA and LA mice did not differ. No genotypic differences were observed in non-dependent mice, discounting possible differences in basal naloxone sensitivity and/or opioid peptide levels. Thus, the two selection projects, while both producing lines exhibiting highly divergent sensitivity to morphine analgesia, have not had analogous effects on all opioid measures, supporting the notion of independent genetic mediation of opioid analgesia and dependence. Further, these data suggest that analgesic sensitivity may not predict sensitivity to morphine dependence.