Inbred strain differences in morphine-induced analgesia with the hot plate assay: a reassessment

Using the hot plate assay of analgesia, several investigators have reported DBA/2J mice to be much more sensitive to morphine and other opioids than C57BL/6J mice using paw-lick as the behavioral end point. In the present studies, we compared DBA/2J, C57BL/6J, and C3H/HeJ mice on two behavioral end points, either (1) the initial response to the hot plate, either a hind paw-lift, paw-shake, or paw-lick, whichever occurred first, or (2) the paw-lick response. In response to either morphine or saline, all three strains showed roughly equivalent latencies to the initial response, but the DBA/2J strain was markedly slow to show paw-lick as a nocifensive response compared to the C57BL/6J strain. As a result, only for the paw-lick response were there significant differences among the three inbred strains in morphine analgesia. Thus, differences in analgesic sensitivity among these strains are largely a function of the behavioral end point used to assess nociception to the hot plate.