Increases in plasma beta-endorphin and tail flick latency in the rat following burn injury.

In children with burn injuries we found, in earlier studies, an inverse association of plasma beta-endorphin immunoactivity (iB-EP) and pain levels. To further explore the effects of burn trauma on the peripheral release of beta-endorphin and the occurrence of centrally mediated stress analgesia, plasma iB-EP levels and tail flick latency (TFL) were measured in rats subjected (while anesthetized) to scald injury. In comparison to sham burn (dip in tepid water), burn injury increased plasma iB-EP and TFL; both the duration and magnitude of these effects were directly proportional to the extent of burns. In rats receiving no treatment, TFLs were unchanged throughout the time of the burn experiments. At 2 days post-burn TFLs were invariably back to pre-burn levels. Administration of the long-acting opioid antagonist naltrexone prior to burn injury prevented the rise in TFL. Thus the trauma of burns appeared to bring about a stress-induced analgesia (SIA). The marked increase in iB-EP during this SIA and its antagonism by naltrexone suggest that it was opioid and hormonal in character.