Strain differences in reversal of conditional analgesia by opioid antagonists.

The ability of both naloxone and naltrexone to block conditional analgesia produced by shock associated contextual stimuli was determined using rats from three suppliers. Naltrexone (7 mg/kg) was equally effective in reversing analgesia on the formalin test in all rats tested. Naloxone (7 mg/kg) significantly affected analgesia in Long-Evans and Holtzman Sprague-Dawley rats but had no effect on Charles River (CD) Sprague-Dawley rats. Charles River rats also differed from the other groups in the amount of time spent freezing to apparatus cues. These results indicate that genetic factors and the choice of antagonists may complicate distinctions between opioid and nonopioid analgesia.