Effects of intraplantar injection of deltorphin (Delt, 30 μg) and morphine (MS, 100 μg) alone and in combination on thermal and mechanical sensitivity in naïve animals. (A-D) Neither drug alone significantly affected any of the behavioral measurements compared to vehicle (saline) injection. (A-B) In contrast, combined deltorphin (30 μg) and morphine (10-100 μg) dose-dependently elevated paw withdrawal latencies (PWL) to thermal stimulation throughout the duration of testing. (A) Time course. *p < 0.05 vs. vehicle. (B) Area under the curve (AUC). *p < 0.05 vs. vehicle, deltorphin alone, morphine alone, and Delt + 10 μg MS groups. (C) Percent withdrawal to a noxious mechanical stimulus and (D) mechanical paw withdrawal threshold (PWT) values were unaffected by combined opioid administration. N = 5-9/group.