Fig. 3. Opioid-mediated (naloxone-reversible) stress-induced antinociception (SIA) produced by tail-clip application in room air, and correlation of strain-dependent SIA magnitude with minimum alveolar concentration to inhibit response to tail clip (MAC). ( A ) Latency to attack (i.e. , attempt to remove) a clip exerting 500 g of force to the tail in mice of 11 strains pretreated with saline ( open bars ) or 10 mg/kg naloxone ( filled bars ). Bars represent mean ± SEM latencies (s). Inset shows the main effect of naloxone in all mice, collapsed across strain ( ***P < 0.001). ( B ) Latency to attack ( i.e. , attempt to remove) a clip exerting 2,000 g of force to the tail in mice of 11 strains pretreated with saline or 10 mg/kg naloxone. Bars represent mean ± SEM latencies (s). Inset shows the main effect of naloxone in all mice, collapsed across strain ( ***P < 0.001). ( C ) Genetic correlation between SIA magnitude ( i.e. , saline-treated strain mean − naloxone-treated strain mean) and MAC using a 500-g clip ( left ) or a 2,000-g clip ( right ). Symbols and regression lines are as in figure 2.