Enhanced potency of receptor-selective opioids after acute burn injury.

Dose-response curves of three receptor-selective opioids were established in a group of nonburned and a group of burned rats. Morphine (mu-agonist), biphalin (mu- and delta-agonist), and U50488H (kappa-agonist) were administered to each group, and analgesia was measured by tail flick latency testing. Each opioid had a significant increase in potency (i.e., a decrease in ED50 values) in the burned (15% body surface area) compared with the nonburned groups. Moderate doses of each drug (i.e., ED50 doses estimated from nonburned group data) in each case augmented stress-induced analgesia in the burned group. Analgesic doses failed to prevent a significant increase in plasma beta-endorphin and corticosterone after larger surface area (25%) burns. Regardless of receptor specificity, opioid analgesic potency is increased acutely after burn injuries.