Effects of combined opioid administration on thermal sensitivity are restricted to the periphery and blocked by antagonists. (A-B) While PWL measurements taken from hind paws injected with combined deltorphin (Delt, 30 μg) and morphine (MS, 100 μg) were elevated throughout the testing period, measurements from the contralateral paw did not vary compared to vehicle at any time point. (A) Time course. *p < 0.05 vs. vehicle. (B) Area under the curve (AUC). *p < 0.05 vs. vehicle and contralateral groups. (C-D) Co-administration of naloxone (Nalox, 100 μg), naltrindole (Nalt, 36.3 μg), or CTAP (30.3 μg) significantly reversed the effect of combined deltorphin and morphine administration. (C) Time course. (D) Area under the curve (AUC). Combined opioid data is repeated from a subset of fig. 1 for comparison. *p < 0.05 vs. Delt + MS group. N = 6-8/group.