Figure 4. Co-administration of IBNtxA and β₂-AR antagonist produces robust synergy in analgesic efficacy.

(a) β₂-AR antagonist ICI 118,551 and 6TM-MOR ligand IBNtxA produce dose-dependent analgesia. Graphs show late phase (10–60 min post-injection) nocifensive behaviors after 5% formalin injection into the plantar hind paw; bars represent mean percentage of 10-sec mice (1 mouse/min) featuring licking behavior directed toward the injected paw (% mice licking). Error bars represent SEM. Left graph shows that the 25, 50 and 100 mg/kg doses of ICI 118,551 significantly inhibit pain behavior compared to saline. Right graph shows that the 1 and 5 mg/kg doses of IBNtxA differ significantly from saline. (b) Co-administration of ICI 118,551 and IBNtxA produces synergistic analgesia. Left graph shows the dose response curve of the cocktail tested), and the right graph shows the isobolographic analysis, confirming the synergistic relationship of these two drugs (p < 0.001). (c) Neither β₁-AR antagonist betaxolol nor β₃-AR antagonist SR 59230A show a synergistic interaction with IBNtxA. Left graph shows the dose response curve of the cocktail tested, and the right graph shows the isobolographic analysis, showing no synergistic relationship of IBNtxA and either betaxolol or SR 59230A (p = 0.90 and 0.49, respectively). *p < 0.05, **p < 0.01, ***p < 0.001.