Preexposure to a nonaggressive opponent prevents low-intensity, social-conflict analgesia in mice.

In a first experiment, exposure of DBA/2 mice to a small number of attack bites by a C57BL/6 mouse resulted in low-intensity analgesia as assessed by the tail-flick test. The analgesia dissipated within 10 min and was insensitive to naloxone (10 mg/kg, sc) but was antagonized by the irreversible opioid antagonist beta-chlornaltrexamine (5 mg/kg, sc). In a second experiment, preexposure to a nonaggressive C57BL/6 opponent prevented low-intensity analgesia induced by a small number of attack bites 24 hr later. The preexposure effect was abolished by naloxone (10 mg/kg, sc) given before the nonaggressive confrontation. This suggests that the release of endogenous opioids during preexposure interferes with the subsequent activation of endogenous opioid-mediated pain control mechanisms.