DIFFERENTIAL SENSITIVITY OF MICE BRED FOR STRESS-INDUCED ANALGESIA TOMORPHINE AND ACEA-1011 IN THE FORMALIN TEST

The antinociceptive effect of morphine, an opioid receptor antagonist, was examined in the formalin test in mice selectively bred for high ( HA) and low (LA) swim stress-induced analgesia (SSIA). A subcutaneous (SC) injection of formalin produced a biphasic nociceptive response in both lines. HA mice spent more time licking the injected paw than the LA mice in both phases of the formalin test. Morphine was equally pot ent in the early phase in both lines, but it was more potent in HA mic e than in LA mice in the tonic late phase of the formalin test. Simila rly, ACEA-1011 produced an equally potent antinociceptive effect in th e early phase in both lines; however, the compound was more potent in LA mice than in HA mice in the tonic late phase of the formalin test. These data suggest that in HA mice antinociception in the tonic late p hase of the formalin test is mediated largely by an opioid-mediated me chanism, whereas in the opioid-deficient LA line at least a nonopioid- mediated mechanism involving the NMDA receptor is also implicated.